mersing the paper in ether. This, however, did not answer. But as it would be tedious to detail all the pains I took to discover what would not do, and to find in what proportions and in what order the right materials could be best applied, I will briefly give the formula which I have adopted, and by which the specimens alluded to were produced: -First, take the white of eggs, and add 25 per cent. of a saturated solution of common salt (to be well beat up and allowed to subside); float the paper on the albumen for thirty seconds, and hang up to dry. Secondly, make a saturated solution of bichromate of potass, to which has been added 25 per cent. of Beaufoy's acetic acid; float the paper on this solution for an instant, and when dry it is fit for use: this must be done in the dark room. Thirdly, expose under a negative, ⚫in a pressure frame, in the ordinary manner, until the picture is sufficiently printed in all its details, but not over-printed, as is usual with the old process. This requires not more than half the ordinary time. Fourthly, immerse the pictures in a vessel of water in the darkened room; the undecomposed bichromate and albumen then readily leave the lights and half-tints of the picture. Change the water frequently, until it comes from the prints pure and clear. Fifthly, immerse the picture now in a saturated solution of protosulphate of iron in cold water for five minutes, and again rinse well in water. Sixthly, immerse the pictures again in a saturated solution of gallic acid in cold water, and the colour will immediately begin to change to a fine purple-black. Allow the pictures to remain in this until the deep shadows show no appearance of the yellow bichromate; repeat the rinsing. Seventhly, immerse, finally, in the following mixture:-Pyrogallic acid, two grains; water, one ounce; Beaufoy's acetic acid, one ounce; saturated solution of acetate of lead, two drachms. This mixture brightens up the pictures marvellously, restoring the lights that may have been partially lost in the previous parts of the process, deepening the shadows, and bringing out the details; rinse, finally, in water, and the pictures are complete when dried and mounted. The advantages of this process may be briefly stated as follows:- First, as to its economy; -Bichromate of potass, at 2d. per ounce, is substituted for nitrate of silver at 58. per ounce. Secondly, photographs in this way can be produced with greater rapidity than by the old mode. Thirdly, the pictures being composed of the same materials which form the constituent parts of writing-ink, it may be fairly inferred that they will last as long as the paper upon which they are printed. A beautiful photograph of Sir Walter Scott's monument, obtained by this process, was exhibited to the Section.

On Moon Blindness. By Sir G. ROBINSON.

The author gave several instances of his men who had slept on deck exposed to the moonbeams being so blind on landing that they had to be led by the hand. Also the sailors were in the habit of waking up the soldiers who attempted to sleep on deck, and warning them that they would be blinded.

On an early form of the Lenticular Stereoscope constructed for the use of Schools. Exhibited by Mr. SAMUEL, Edinburgh.

It consists of two semilenses on a frame, from the middle point between which there extends a brass rod upon which the stereoscopic picture slides, so as to have two adjustments; one along the rod for persons of different ages; and one of a rotatory kind, to place the horizontal lines in the picture parallel to the line joining the two eyes, an adjustment which exists in no other form of the stereoscope. This instrument is described in Sir D. Brewster's 'Treatise on the Stereoscope,' p. 69.

On the Ocular Crystal Micrometer, with observations of twelve double stars, as evidence of its extraordinary power in measuring small angular distances. By NORMAN POGSON. (Communicated by JOHN LEE, LL.D., F.R.S.) The instrument which it is the object of this paper to recommend to the notice of astronomers, is of such easy application, yet capable of yielding such extraordinarily

accurate results when applied to the measurement of small distances, that it appears singular, as well as a matter of regret, it should so long have awaited a fair trial, and the publicity it so well deserves.

Although copiously described by Dr. Pearson in his valuable work on 'Practical Astronomy,' so many varieties of double-image and other micrometers are there treated, that the one under consideration has failed to attract more than a passing notice from the Doctor's readers, and unfortunately no published results have made their appearance from which amateurs might judge of the accuracy attainable by its use. Had such been forthcoming, doubtless the Ocular Crystal Micrometer would have been generally adopted, and the well-grounded complaint, of the inferiority of the measures of distance of double stars to those of the angles of position, would never have been heard of.

Perhaps the best method of drawing attention to this micrometer will be briefly to explain the construction and use of one, originally made for Admiral Smyth,transferred by him to the Hartwell Observatory in 1829, on becoming possessed of the larger one described in his 'Celestial Cycle,' and recently placed in the hands of Mr. Pogson for trial, by the kindness of its present owner, Dr. Lee.

The Ocular Crystal Micrometer consists of a variable eyepiece, i. e. one in which the second or field-lens is moveable by a rack-work, so as to vary the distance between it and the eye-lens. This distance is read off, by the help of a vernier, on a scale of equal parts, and this, as will be shown, is an important element in the observation. By a well-known optical formula, if e, f, and O represent respectively the focal lengths of the eye-lens, field-lens, and object-glass of a telescope, when used with a certain eyepiece, also d, the distance between the first-named lenses, the magnifying power of the telescope with such eyepiece will be thus found :

From this it is manifest, that when the two lenses are in contact, the power will be a maximum; that as the distance between them increases, the power will diminish; and that the equal divisions on the scale which records this distance will, when multiplied by the factor give the corresponding changes in the magnifying power.

0 e.f

It is therefore sufficient to determine, with a dynameter, the magnifying powers when the lenses are in contact, and when most widely separated, and by simple proportion to tabulate the intermediate divisions and corresponding powers. By having two or three eye-lenses, which can be slipped (not screwed) into their cells, the range of powers is very considerably increased. Thus in our micrometer, eye-lens No. 1 extends from powers 261 to 134; No. 2, from 135 to 78; and No. 3 from 71 to 48.

To produce a double image, two prisms of rock-crystal-the one cut in the direction of its optical axis, the other transversely thereto-are cemented together, 80 25 to form an achromatic solid of double refraction. Six such prismatic solids, of constant angles from 2474" to 192", are in our micrometer fitted into brass caps, which are made to slip on, in front of the eye-lens; so that if the separation of the images is too great, the prism can be immediately changed for another of a less constant angle, and vice versa. The double image is therefore formed after the telescope has performed its office, and is much cleaner and more distinct than in the original contrivance of Rochon, the inventor, who placed his prisms between the eyepiece and the object-glass. No wings or coronæ trouble the observer as with divided eye glass micrometers; and although of course half the light is lost by the duplication of the image, the tedious additions of clock motion and illumination of the field are dispensed with. Indeed, the loss of light sustained by the use of illumination barely sufficient to render the spider lines of a wire-micrometer visible, far exceeds that occasioned by the transmission of the rays through good prisms of rock-crystal. Perhaps the whole secret of the excellence of this micrometer lies in the position of the crystal being BEFORE instead of behind the eyepiece.

On viewing a double star through the Ocular Crystal Micrometer, two pairs will be seen, the four members of which must be brought into the same straight line by turning the crystal round in its cell. The measure is then made by varying the

magnifying power, i. e. the distance between the two lenses, until the four stars appear equidistant. If the power be too small, the middle space will be greater than that between each pair of stars; if too great, the reverse. When the four are, however, satisfactorily adjusted and the scale read off, the measured angular distance is equal to the constant angle of the crystal divided by the magnifying power employed. By placing the four stars at equal intervals, the double distance will be measured, and the uncertainty of a contact between two perhaps very unequally bright stars avoided, which is highly desirable. It is therefore of the utmost importance to know the constant angles of the prisms, also the limits of magnifying power of the variable eyepiece, with all possible accuracy. It does not enter our purpose here to discuss the various methods recommended by Dr. Pearson for determining these instrumental constants; suffice it to remark, that when once found they are not liable to change, and their investigation is by no means difficult.

The observation of position angle, though less certain than that of distance, is at least equal to what can be made with any other kind of micrometer. For this purpose a fine diametral line is cut across the flat side of the field lens, which becomes visible when screwed into the focus of the eye-lens. This line must be adjusted by running the double star, or if more convenient any adjacent bright one, along its entire length. This done, the prism, which is attached to the vernier of the position circle, must be moved round until the two images of the line are seen in coincidence, when the reading will be the zero-point of the position circle. For these two operations the field requires illumination, though not for the absolute measures. The four stars seen through the prism must next be brought into the same straight line, as in the measure of distance, when the difference between the circle-reading and the zero previously found, plus or minus ninety degrees, will give the angle of position. Unless the equatorial adjustments of the telescope have been very correctly made, it is better to take half the number of zero readings before, and half after those of position, so as to eliminate any change of the zero arising from the alteration of hour-angle during the observation.

A temporary defect in our micrometer, which could not be remedied without parting with it just at the time most convenient for its use, prevented the observations of angle of position being made. A few only were attempted, but sufficient to show that good results may be expected in this coordinate also when the micrometer is put into working order. But as the observations appended are to be regarded merely as examples of its capabilities as a distance-measurer, under unfavourable conditions, and by no means in the light of a series of scientific results, it would not be rendering justice either to the instrumentor to the observer to add the few imperfect angles of position obtained, and they have therefore been omitted.

No kind of micrometer is so suitable to, or convenient for, an unprofessional astronomer, who cannot enjoy the luxuries of an observatory; and it is no exaggeration of its merits to say, that any good portable telescope, so equipped, will enable its possessor to compete successfully with far larger instruments in fixed observatories not so provided. To prove this point, it is only necessary to refer to the annexed observations of well-known double stars, measured on different nights, frequently with different crystals, and therefore perfectly free from any pre-occupation of mind. As first attempts with a strange instrument, they are naturally open to great improvement; but with practice and care, it seems reasonable to hope that differences exceeding half a second, or for small distances a twentieth part of the measured space, will rarely, if ever occur.

It is a striking proof of the excellence of the Smythian telescope, that with an aperture of only 3-6 inches, with the light diminished by the duplication of the image, and by its transmission through tolerably thick prisms of rock-crystal, powers over 200 were so readily usable. Its definition is beyond all praise : without the prisms, it bears a power of 400 well, and separates the closer double stars in a manner which puts to shame many much larger telescopes.

Date. 1858.

Distance Measures of Twelve Double Stars with the Ocular Crystal Micrometer.

Sept. 4. 51233-3 3-42 5 Aug. 12. 15 | 89-6 | 8-915 Aug. 3. 6 122-3 10-11/3

5. 5 234-4 3.40 5

13. 5 89-2 8-95 5

"

9.6 123-9 9-995

Aug. 8. 6 226-25-47 5 Aug. 3. 6 85-6 14-44 3 Aug.

9.5 144-9 5.51 5

3.5 55-1 14-50 3 Sept.

5.6 86.9 14-23 5

1. 6 255-3 4-85 5 4. 4 112-3 4-57 5 5.6 256-6 4-82 5 6. 4 111-44-61 5

Aug. 1.5 122-1 6-53 3 Aug. 9. 6 88-7 13-95 5 Aug. 1.14 171-2 3-00:2

Aug. 5. 4 202-3 2-54 5 Aug. 1. 6 109-111-341 Sept. 4. 5 231-2 3-45 5

% Aquarii.

Measures.

To arrive at a general conclusion as to the accuracy attainable with this micrometer, agreeably to the sixty-five observations here given, we shall find that for the mean power 152.7, and mean distance 767, the probable error of one observation, based upon five measures, will be 0.086.

Also, if we suppose the accuracy to increase in direct proportion with the power, the extreme difference to be expected amongst five such observations, taken with a magnifying power of 200, will be 0-21.

On the Distribution of Heat in the Interior of the Earth.
By Dr. F. A. SILJESTRÖM, of Stockholm.

Both the plutonic and volcanic phenomena are generally ascribed to causes residing within the earth's solid crust, whilst the interior fluid mass is taken into consideration only with regard to what Humboldt calls its "reaction." Without disputing this way of explaining the phenomena-though I really think it liable to several grave objections-I wish to call attention to a cause of dilatation (and contraction) that certainly exists, or at least has existed within the fluid mass itself, and which, I think, must be considered as an important item in this question.

It seems impossible originally not to suppose different temperatures in different parts of the fluid earthy mass. Considering the great absolute temperature, as well as the immense bulk of the earth and other circumstances, differences as great even as 100° C., nay more, can in no way be regarded as improbable. The natural consequence thereof was the formation of currents, by means of which differently heated parts were brought together, and the temperature of the mass was made more and more uniform. However, on various grounds I conclude that even now the temperature cannot be one and the same through the whole fluid nucleus of the earth, but that currents of the said description still exist in the interior of our planet.

This assumed, let v, v be the volumes, and t, t the temperatures of two differently heated fluid parts, which are mixed together, and which, for more simplicity, may be regarded as having the same mass and as being of the same chemical nature. Let, further, w be the volume and T the temperature of each after the mixture. As both the dilatation and the specific heat change with the temperature, let A, e be the mean dilatation and the mean specific heat between the temperatures t and T, and A', e' between T and t (t being > t'). Hence it follows that

Now +' being the original volume and 2w the volume after the mixture, it will be seen that there must needs be a change of volume, unless Ae'=∆'e, which at least is not the case with the substances, for which Dulong has determined the variations of dilatation and specific heat.

1

If we take as an example the values found by Dulong for iron between 0°-200°, it will be seen that if only 1,000,000 of the earth's volume were subject to the abovenamed process of mixture-one part being considered 200° warmer than the otherthe result would be a change of volume certainly not less than five or six times the whole bulk of Vesuvius. This may in some way be illustrative of the quantity of action that might be supposed. As to the absolute intensity and the mode of working, I think no other force could be imagined more suited to the purpose.

An Account of some Experiments on Radiant Heat, involving an Extension of Prévost's Theory of Exchanges. By B. STEWART.

These experiments were performed with the aid of the thermomultiplier, the source of heat being for the most part bodies heated to 212°. Four groups of experiments were considered. Group the first contains those experiments in which the quantities