every case, it is stimuli that are mixed and not sensations; that the mixture is of physical or physiological processes, not mental processes. A "mixed" color is as simple, psychologically, as any other. Visual sensation depends not only upon the light stimulus to which the eye is exposed and upon the mixture of stimuli, it depends also upon (a) the part of the retina stimulated, (b) the stimulation of adjacent areas, and (c) certain more or less permanent effects left by the stimulus upon the visual organ. (a) It is only in the central part of the retina the part lying about the fovea or spot of clearest vision that all the colors are seen; further out toward the periphery of the retina lies a zone in which reds and greens are not sensed (only blues and yellows), and, still further out, an outermost zone whose stimulation gives rise to brightness sensations, only That is to say, the normal eye is totally color-blind over a part of its sensitive area and partially color-blind over a second part. Abnormal colorblindness, then, which is relatively common in the race, may be regarded - at least in its commoner forms- -as an extension to the fovea of the normal color-blindness of the normal eye. The eye is a chemical sense; that is, chemical processes are interpolated between the reception by the eye of light-waves and the neural processes that are ultimately set into function by them. Two special results follow from this fact. First, stimulation is not strictly confined to the part of the retina directly affected by light; and, secondly, nervous excitation continues after the external stimulus has ceased to operate. (b) The first of these results, retinal irradiation of chemical or photo-chemical processes, is strikingly demonstrated by the phenomena of contrast. A patch of white looks whiter if it is placed on a black surface; black looks blacker in the neighborhood of white. In a similar manner, the saturation of a color (for example, red) is enhanced if it is brought near a complementary color (blue-green); and, finally, a gray in the immediate neighborhood of a color (for example, green) is tinged, under certain conditions, with the complementary of that color (purplish red). The last case, the induction of a contrast color upon a gray, may be observed in the shadows of snow, which often look blue under yellow sunlight. The general effects, both of brightness and of color contrast, are, moreover, frequently to be seen in clothing and in interior decorations. The fundamental law of contrast is that the contrast effect is always in the direction of the opposite brightness or of the complementary color. (c) The result of continued stimulation of the eye is to change the condition of excitability of that organ. If large colored glasses be worn before the eyes for several minutes the unnatural hue of objects, which is at first very noticeable, gradually disappears. With blue glasses, objects at first look bluish, but finally return to their proper tones. The eyes are said to have become blue-adapted. A similar effect is produced if only a small part-not the whole of the retina is exposed for some moments to colored light. A piece of red paper, for example, hung on a gray wall and fixated steadily gradually loses its saturation and approaches a gray. But not only is the excitability of the eye altered under continued stimulation; it remains altered, are as noted above, even after the exciting cause has ceased to operate. When the blue glasses are removed the landscape looks yellowish; the observer is yellow-sighted. Similarly, when the red paper is removed, a bluish-green patch appears upon the gray wall. The first result is due to general adaptation, the second to local adaptation. In both instances, adaptation forms a predisposition for seeing colors which complementary to the original stimulating colors. The result of local adaptation (as the bluishgreen patch induced by the reddish paper) is a negative after-image. Adaptation and afterimages follow the same course with brightnesses as with colors. It is, for example, a matter of common observation that confinement in a darkened room enhances the brightness of objects seen subsequently in full light; that exposure to a strong light (sunlight on snow) tends to reduce the apparent brightness of objects seen afterward in moderate illumination, and, finally, that fixation of a dark or a light area induces a negative after-image of the opposite brightness. All these phenomena illustrate the effects of adaptation. Theories of Visual Sensations. These theories aim to set forth the conditions under which the sensations arise. The most important in the field are the rival theories of Hermann V. Helmholtz and E. Hering. The Helmholtz theory provides for three primary sensations, red, green, and blue (or violet), which rest upon three distinct processes of excitation, in the visual apparatus. To explain the large number of spectral qualities, it is assumed that these three processes, combined in varying proportions, give rise to all possible color qualities, and that in equal amounts they produce gray. The theory, which was outlined by Thomas Young and elaborated by Helmholtz, was designed primarily to account for the facts of color-mixture; that is, for the production of a large number of "intermediate» qualities and of grays by means of a few "elementary" sensations. These facts of color-mixture it covers well; but it fails - particularly in its traditional form-to explain many other facts of color vision, notably the possibility of obtaining gray in the absence of color tone (as in color-blindness, peripheral vision, and from stimuli of small extent). Recently, important modifications and additions have been introduced into the theory which have, in a measure, removed its deficiencies. The Hering theory rests upon a basis quite different from that of the Helmholtz theory. Hering posits three retinal substances; but he makes each of these the seat of antagonistic processes-processes of assimilation or building up and of dissimilation or tearing down. To each of these processes in each substance corresponds a "primary" color. Assimilation of one substance gives rise to blue; dissimilation of the same substance, to yellow. This substance is called briefly the blue-yellow substance. Similarly the other two substances are called the red-green substance and the black-white substance. Blue, green, and black correspond to the three assimilative processes; yellow, red, and white to the three dissimilative processes. All forms of stimulation affect the black-white substance; but certain wave-lengths produce no effect upon the other two substances. over, the black-white substance appears in great More est quantities, is most widely distributed throughout the visual apparatus, and is most easily set into function. Since assimilation and dissimilation are opposed processes, the principle of antagonism assumes great significance in Hering's theory: the principle is, indeed, its most characteristic feature. Its application to complementary colors - where the opposed processes in the several substances cancel each other - to contrast, to adaptation, to after-images, etc., follows naturally from the fundamental conception. Although the Hering theory is open to attack on various technical grounds, it undoubtedly covers the whole range of visual sensation more adequately than its rival. There is a good deal that is hypothetical in both theories. This is, however, a fault that recent work on the histology of the eye and on the function of the retinal elements is doing much to remedy. Within the last few years several new theories of visual sensation have come into the field. Most of them may, however, be regarded as modifications of one or other of the more classical theories just discussed. Their value and their relation to the older theories must be sought in the current literature of the subject. Visual Perception. It is characteristic of visual perception that all objects seen are spatial objects; that is, they occupy some place, some position in the spatial world; and they possess, likewise, spatial properties, form, distance, direction, etc. This characteristic, it should be noted, is shared by tactual perceptions; but it is not a mark of perception in general-not, for example, of the perceptions of melody, harmony, and rhythm. Since all visual perceptions are spatial, the chief problem they offer to psychology is the analysis of the spatial factors and the search for the conditions under which these factors operate. object. Now it is conceivable that the two unlike images should have come to consciousness as two distinct objects. But, as a matter of fact, they do not, except under unusual circumstances. as when one squints or presses with the finger upon one eyeball. They do not even come to consciousness as two discrepant views of the same object. The difference in retinal images functions solely in the perception of depth or solidity. This is the principle involved in stereoscopic vision. Binocular perception of depth may be produced artificially by means of the stereoscope, an instrument which presents to the two eyes, under favorable conditions, two slightly different plane pictures of an object. Stereoscopic vision derives secondary aid, in ordinary perception, from linear and aerial perspective, from the known size of objects of reference, from change of position of the observer as well as of objects, from distribution of light and shade, etc. well upon ocular movements and the sensations The spatial functions of the eyes depend as which they arouse as upon the immediate retinal strated, by many lines of experimentation, that factors just considered. It has been demonwithout the possibility of movement (a condiwere the eyeballs set firmly within the head tion which is approximated in the case of cerbe enormously handicapped. In the first place, tain fishes) visual perception of objects would movement allows the eye to travel over the object, exploring it from point to point; in the second place, it sets up sensations both in the external muscles that rotate the eye and in the internal muscles of accommodation; and finally, it is probable that to these factors must be added articular sensations from the rubbing of the eyes in their sockets. All these sensations, muscular, tendinous, and articular, play important roles in the determination of the spatial properties and relations of objects. The eyes, regarded as a perceiving organ, may be said, then, to fall into three parts; the retinas, which mediate visual sensations proper, and which function as a true double organ, the dioptric media, which focus the rays of light upon the retinas, and the movement-apparatus, which both alters the position of the retinal image and itself contributes kinæsthetic sensations from muscle, tendon, and orbit. Consult: Calkins, Introduction to Psychology) (1901), ch. ii.; James, Principles of Psychology (1896), Vol. II., ch. xx.; Kuelpe, Outlines of Psychology) (1895), pp. 351-373; Stout, 'Manual of Psychology) (1899), pp. 141170; Wundt, Human and Animal Psychology); Titchener, Experimental Psychology,' Vol. I. (1901), Part II., ch. i. For the structure and functions of the eye, consult also general textbooks of physiology. See EYE. The simplest factor in visual space is extension. Every visual sensation comes to consciousness as an extended sensation. A color is always "spread out"; its parts lie side by side. In this respect, colors and brightnesses are essentially different from tones and noises, which lack the attribute of extension. Space, as it is perceived, is an orderly arrangement of extended objects. It never exists by itself alone. There is no such thing, in perception, as "mere" space or "empty" space. Only by abstraction are the spatial properties removed from objects; only by abstraction, as in mathematics, does empty space come into existence. Even such quasiabstract constructions as the line and the point do not properly figure in the psychology of space. It is, however, customary, within psychology, to distinguish two-dimensional and three-dimensional spaces; not because they differ fundamentally, but because they rest in part upon different conditions. The retina, being an extended organ upon which stimuli fall in patterns - thanks to the refractive functions of the transparent media Asst. Professor Psychology, Cornell University, - forms a natural substrate for the perception of surface magnitudes. But the retina is not so well adapted for tridimensional perception - perception of solid objects. A special provision for depth-perception is made, however, in binocular vision; that is to say, in the difference of the two retinal images which are thrown upon the two eyes by a single object. This difference in binocular images depends upon the different positions from which the two eyes observe the I. M. BENTLEY, common of Vision, Defects of. The conditions classed as defects of sight or vision are mainly due to anomalies in the structure of the eye, causing errors of refraction and other visual irregularities. Among the more these defects is that of astigmatism (q.v.), which is usually characterized by a symmetry in the curvature of the cornea (q.v.) in different meridians. Color-blindness (q.v.) is a serious disability in certain occupations. Day-blindness VISION OF SIR LAUNFAL VISITATION (nyctalopia) is a condition in which one sees better in a dim light than in a bright light, due to some opacity in the cornea, the crystalline lens, etc. It occurs in amblyopia (q.v.) and other affections which produce dimness of vision. Night-blindness (hemeralopia), on the other hand, is a state in which the person affected has normal vision in an ordinary light, but in twilight becomes suddenly blind. Double vision (diplopia) occurs when, as in some cases of squinting, each eye sees things separately. This defect arises from derangement of the visual axis, sometimes through muscular paralysis. In longsightedness or farsightedness (hyperopia or hypermetropia) objects are seen distinctly only at a range beyond that belonging to normal vision. Owing to the shortness of the eve-cavity the lenses are unable to converge the rays to a focus within the limits of the eyechamber, the image being therefore formed (theoretically) behind the eye. This defect is corrected by the use of convex lenses, which by converging the rays of light cause the image to fall on the retina. Shortsightedness or nearsightedness (myopia) is the reverse of longsightedness in causes as well as in effects. In this, owing to the too great power of the crystalline lens, or to the extension of the eye-cavity too far backward. images from objects at some distance are formed in front of the retina, mak ing the sight confused, if not entirely defective, for things beyond a certain limited range, while rendering it very clear for near objects. For remedying this condition biconcave glasses are employed, which, unless the myopia is serious, need be used only for looking at objects far off. Shortsightedness and longsightedness are usually congenital. Presbyopia is a defect similar to hyperopia, and usually comes on with advancing years, naturally beginning from the 40th to the 45th year It is due to diminished focusing power and lessened elasticity of the lens, the result of which is that the image of a near object is not clearly formed on the retina, but is formed behind it, while distant objects are seen as well as ever. Convex lenses are used for remedying this condition. Strabismus or squinting is a deformity often seen, and is ascribed to want of parallelism in the visual axes when the effort is made to direct both eyes to an object at the same time. It may be due to loss of power (paralysis) of one or more of the eye muscles; and this may depend on a merely local affection, or may be a symp tom of serious brain disease. But in the majority of instances and in all ordinary cases no such condition is present. The squint is said to be convergent when the squinting eye is directed toward the nose, and divergent when it is directed toward the temple; the convergent is much the more common. Concomitant strabismus is a variety of which the amount continues about the same in all positions of the fixationpoint. When the direction of the eye or eyes is upward or downward the squinting is said to be vertical Convergent squint usually comes on during childhood, most often from the 2d to the 7th year. It is sometimes due to defective sight in the squinting eye, from congenital abnormality, severe inflammation, or injury; but very often no such condition is present. In a large proportion of cases it is accompanied by hypermetropia, and is due to the increased effort of accommodation required to see near objects, being associated with an increased and disproportionate effort of convergence. In such cases, if suitable glasses can be worn as soon as the squint begins to show itself, it may be prevented from becoming permanent. In other cases the presence of a squint may be traced to worms, the irritation of teething, etc.; and it disappears when the cause is removed. Divergent squint is very rarely present without considerable defect in the sight of the squinting eye, except where it is the result of over-correction of a convergent squint by operation. It is often associated with myopia, as the other form is with hypermetropia. The surgical operation for the correction of a squint consists in the division of the muscle whose excessive activity leads to the faulty position-in convergent strabismus the internal rectus, in divergent the external. It is often necessary to operate on both eyes in the same manner, even where the squint is monocular. In some cases it is requisite in addition to shorten the opposing muscle. See EYE; OPHTHALMIA; SENSES; VISION. Vision of Sir Launfal, län'fal, The, a poem by James Russell Lowell, founded on an Arthurian legend and published in 1845. Vision of Mirza, mèr'za, The, a famous allegory by Joseph Addison, which appeared in No. 159 of The Spectator.' Vision of Piers Plowman, pērz plow'mạn, The, an English poem of the 14th century, ascribed, chiefly on the ground of internal evidence, to William Langlande (q.v.) or Longland, a monk of Malvern, in spirit a Carlyle of the Middle Ages, crying out against abuses, insisting upon sincerity as the first of virtues. This poem belongs to the class of the dreampoem, and reflects both the England of the 14th century and the visionary, child-like mediæval mind. Internal evidence fixes its date about 1362. Forty manuscript copies of it, belonging for the most part to the latter end of the 14th century, attest its popularity. Three distinct versions are extant, known as Texts A, B, and C. The probable date of Text A is 1362-3; of Text B, 1376-7; of Text C, 1398-9. The variations in these texts are considerable. An imitation of the poem, called 'Piers Plowman's Crede,' appeared about 1393. The poet's vocabulary is similar to that of Chaucer, and several dialects are combined in it, the Midland dialect dominating. The metre is alliterative, long lines, divided into half-lines by a pause. Each line contains strong, or accented, syllables in fixed number, and weak or unaccented syllables in varying number. Consult Jusserand, Piers Plowman: a Contribution to the History of English Mysticism' (1893). Visita'tion, in an ecclesiastical sense, the visit of inspection by a bishop, archdeacon, elder or other authority to the churches within his jurisdiction. In the Protestant Episcopal churches the term is applied to an annual assembly of clergy and church-wardens, for the purpose of admitting church-wardens elect to office, reviewing the condition of the parish, answering inquiries relating thereto from the superior ecclesiastical authority, and receiving a charge from the bishop or his representative. Under the church establishment in England, the annual visitation is of an official as well as religious VISITATION NUNS-VITAL STATISTICS character, and acts of Parliament bearing on church affairs are then formally communicated to the clergy and laity. Visitation Nuns. See ORDERS, Religious. Visitation, Right of, the act of a naval commander who visits a ship of another nation to ascertain her character, and the object of her voyage. In time of war a search is likely to follow if the replies to inquiries are not satisfactory. Vistula, vis'tu-la (German, Weichsel), a river of Central Europe, about 650 miles long, navigable from Cracow. It rises on the frontiers of Galicia and Austrian Silesia, on the northern declivity of the Carpathian Mountains, flows first eastward past the town of Cracow, forming from a point about 15 miles below Cracow, the boundary of Galicia and Poland, till it receives the San on the right bank, when it turns northwest through the Polish provinces, enters West Prussia a little above Thorn, and after traversing that province divides into two branches, of which the eastern, the Nogat, empties into the Frisches Haff; the western divides again about five miles from the Baltic into an eastern and a western branch, the latter, the Dantzic Vistula (which is much the larger), proceeding toward Dantzic, the former to the Frisches Haff. A new channel cut since 1895 from the head of the Dantzic branch now carries most of the water directly to the Baltic. The navigation of this river is important, especially to Poland, though often obstructed by ice or shallow water. The canal of Bromberg connects the Vistula, through the Netz and Warthe, with the Oder. Several navigable rivers empty into it. The area of its basin is about 76,000 square miles. Vita Nuova, vē'tä noo-ō'vä (It. The New Life), an early work by Dante, completed most probably in 1307. Vital Statistics may be defined as the science of numbers applied to the life history of populations. The term "demography" is nearly synonymous with vital statistics. The principal factors which are concerned in the study of vital statistics are the number of the population, and the number of births, marriages and deaths which occur in such population. To these may be added statistics relative to the prevalence of disease. The facts which are collected from different nations, communities, and populations, and which are embraced in the term "vital statistics" constitute the basis of definite and accurate knowledge upon many of the fundamental principles of public hygiene, sociology, and of preventive as well as of therapeutic medicine. Among the important data which are contributory to the vital statistics of a community are the age, sex, time of year, race, occupation, and the cause of death. Vital statistics also furnish the necessary data for lifeinsurance calculations. Population. Knowledge of the number of the population living in a given community is essential to accuracy of conclusions as to the vital statistics of such community. For example, the intensity of the birth- or death-rate of a district cannot be compared with that of another district, unless the number of the population living in each is known with a fair degree of accuracy. The number of the population living in a given community is determined by a census or counting of the people. In most coun Vol. 16-31 tries this is performed once in ten years, but for the sake of greater accuracy, in some of the United States, an intermediate or five-year census is taken. Various methods are employed to estimate the number of the population in intercensal years. The method employed by the registrar-general of England, and by many other authorities, assumes that the same rate of increase has prevailed as that which existed between the two immediately preceding census the same as is employed in computing compound enumerations. This geometric rate is practically interest, and assumes that the population increases, and that the increase increases at the same rate. The arithmetical method assumes that a fixed and definite addition is made to the population in each year. Either of these principles may be applied to decreasing as well as to increasing populations. for computing populations in intercensal years: The following formula will be found useful Let P population in a census year 1890. Let P'population in a census year 1900. Let R rate of increase. ing at is Then PP+R10 in the 10th year follow If we begin with population P, the population any following year = P(1 − r) n ̧ Log P1 Log P+ 10 log R. 1/10 (log P1-log P) = log R. Application:- A city of 500,000 has increased to 595,430 in the ten year period 1890-1900: what its estimated population in 1905? Log 595,430.7748307 Log 500,000.6989700 Log Р 1905.7748307 +5(.0075861)= .8127610 log of 649,772 estimated population of city in 1905. In a city or a country like New Zealand, or the city of Berlin, where a strict account of births and deaths is kept, together with the number of immigrants and emigrants, the population can readily be found by balancing the natural increase due to the excess of births over deaths, and the increase or decrease due to migration. and and Registration of Births, Marriages, Deaths.- Records of births, marriages deaths are now compulsory in most European countries, and in some of the States of the Union. The States having such registration are the six New England States, New Jersey, Michigan, and New York. Other States are also preparing for similar work. Massachusetts was the first State to adopt a complete system of registration in 1842. The census bureau at Washington and the American Public Health Association have also taken a lively interest in aiding and forwarding this work. Registration of Disease and Sickness.- In some countries and States, and in most cities, registration of certain infectious diseases which are dangerous to the public health is compulsory, and by this means information is gained as to the spread of such diseases, and as to their fatality. This is accomplished by a system of notification to the health authorities. Births, Marriages, and Deaths are commonly stated as a rate per thousand of the living population. In the case of births a more accurate result would be attained by comparing the VITAL STATISTICS births with the number of women of child-bearing years. If it be desired to make a further classification the legitimate births may be compared with the number of married women of child-bearing years (15-45), and the illegitimate births with the unmarried women of the same ages. The birth-rate varies in different countries from as high as 45 per 1,000 inhabitants in Hungary to as low as 22 in France. Several countries in Eastern Canada occasionally have birth-rates as high as 55-60 per 1,000 inhabitants. The birth rate of the New England States for the 10 years 1892-1901 was 24.7 per 1,000, the maximum for the same period being that of Masachusetts (27.1), and the minimum that of New Hampshire (20.5). The birth-rate has diminished in most countries having registration, that of England having fallen from 36.3 per 1,000 in 1876 to 28.4 in 1903. The causes as stated by Newsholme, being the "postponement of marriage to more mature years," "a larger proportion of celibacy" and mainly by "the deliberate and voluntary avoidance of child-bearing on the part of a steadily increasing number of married persons." The births of males are in most countries from 4 to 7 per cent greater than those of females. Still-births are usually excluded from the birth-rate, since they add nothing to the living population. They are also excluded from the death-rate. Still-births are usually from 2.5 to 4 per cent of all births. The still-born males are about 50 per cent greater in number than the still-born females. Plural Births.-The ratio of plural births in different countries is fairly constant, when large numbers are considered. During the 10 years 1874-83, in seven large countries of Europe (France, Italy, Austria, Prussia, Bavaria, Switzerland and Sweden) in 20,000,000 births the cases of twins were about 12,000 per 1,000,000, and those of triplets about 140 per 1,000,000. Marriages.- Marriages are usually stated numerically as a ratio per 1,000 of the total population, for example, 9.5 per 1,000, which is also equivalent to 19 persons married per 1,000, the number of persons married being twice the number of the marriages. A more accurate method would be to reckon the marriage-rate as a ratio of the number of unmarried persons living, of marriageable ages. The marriagerate is usually higher in the cities than it is in the rural districts. Conditions which affect the marriage-rate are: (1) High or low cost of living; (2) Unequal distribution of the sexes at marriageable ages; and (3) Prolonged war. Deaths. The death-rate is also usually stated as a ratio of the living population, the unit of time being a year. This unit is employed when death-rates for shorter periods are stated. In many large cities a weekly or monthly bulletin is issued in which monthly or weekly death-rates are given; they represent the number who would die out of each 1,000 of the population, supposing the same ratio of deaths to continue throughout the year. The following formula may be used for obtaining this weekly annual death-rate. The approximate number of weeks in a year being 52.18, if the population of a city is 250,000 and the number of deaths in a given week is 85 then the deathrate may be obtained as follows: Correction of the Death-rate.- The deathrate of a city is obtained by taking the ratio of the deaths to the population; but, in many cities. there are large institutions in which many deaths occur of persons who are not residents of such cities. There are also deaths of residents which occur in other places, while the persons are away from home. Correction should be made for such deaths. For example, in 1902, 600 persons died in institutions of Boston who were not residents, and their deaths should be credited to their proper places of residence. Again, the relative numbers of the population at each age differ much in different cities, and this fact materially influences the death-rate. If there is a large percentage of persons at the ages of 15 to 50 when the death-rate is not above 8 or 9 per thousand, then the general death-rate will be low, but if there is an excess of children under 5 and of old persons over 70 then the general death-rate will be high, other conditions being equal. It is customary in English cities to make corrections for these conditions, using the deathrate of the country at different ages as a standard of comparison. ferent ages, as well as for the two sexes. The Age and Sex.- The death-rate differs at diffollowing table shows the death-rate by ages and sexes for England and for Massachusetts: Infant Mortality. The death-rate of infants under one year old is usually expressed for the sake of accuracy as a ratio of the births. Infant mortality varies under different conditions from as low as 90 per 1,000 births up to 300 or more in different countries. The infantile death-rate of manufacturing cities, and especially of those which have a densely crowded tenement-house population, is greater than that of other cities, while that of rural districts is comparatively low. The infantile death-rate of the densely settled State of Rhode Island in the 10 years 1892-1901 was 165, while that of the more sparsely settled State of Maine in the same period was only 120. The following table presents the infant mortality of certain countries for different periods: |