of the focusing glass, the water-mill was turned on and the dark slide containing the sensitive collodion plate was substituted for the ground glass. Mr. Hartnup then took his station at the finder, and, with a tangent rod in each hand, by a steady and continuous movement, kept the point of intersection of the cross wires stationary on one spot on the moon's surface.

When the motion was most perfectly neutralized, I uncovered the sensitive plate at a given signal and exposed it, counting the seconds by means of a loud-ticking chronometer by my side.

From the ease with which on my first attempt I could keep the cross wires of the finder fixed on one point of the moon by means of the tangent rods, I confidently believe that with the well-tutored hands and consummate skill which guided this noble instrument, the moon's image was as motionless on the collodion plate as it could have been were it a terrestrial object.

The negatives which I obtained by these means were exquisitely beautiful, and so minute that I could not obtain paper with a sufficiently fine surface whereon to print copies which would do them justice. It was evident that they would bear magnifying several diameters and still remain sharply defined. The expense of carrying out this design here stopped me, when by the kind advice of Professor Wheatstone I applied to the Royal Society, whose munificence has so frequently been the cause of bringing to a successful termination investigations of the highest importance.

A half-plate photographic combination of lenses, by Ross, was screwed the reverse way into a large sliding camera body 10 inches high by 11 inches wide, and capable of sliding from 18 inches to 3 feet long. At the end of the sliding body opposite to the lens, was a groove to admit either a focusing glass or a dark slide for the sensitive plate. A smaller camera body was screwed into the other end of the brasswork of the lens, having also a groove in front to admit of a sliding box capable of holding the small negatives. A reflector was placed in front of all, so arranged as to move in altitude round a centre, and, being fixed in any required position, to reflect the diffused light of the sky through the negative and lens parallel with the axis of the latter.

Preliminary trials showed me that there was no good gained by magnifying the small pictures more than about 20 times, as after

that the individual parts begin to get confused and indistinct; this magnifying cannot, however, be effected at once. In the small negatives the lights and shades are the reverse of what they are in nature, consequently a print on paper therefrom gives the light and shade correct. A photographic copy of a negative, however, produces a positive by transmitted light, and a print from this would have the shadows light and the light parts dark; consequently, in magnifying a negative with the intention of still producing a negative, an intermediate transmitted positive must first be taken, and this in its turn magnified, when it will produce a negative.

The relative distances of the negative and focusing glass from the intermediate lens were so adjusted, that an image of the negative, enlarged to about two diameters, was thrown upon the ground glass, care being taken that the light from the sky was reflected parallel through the centres of the negative and lens by means of the mirror. The aperture of the lens was then stopped down to half an inch by means of a diaphragm, and the focus most carefully obtained by sliding the end of the large camera in or out. I found it necessary to verify this by experimental trials at different distances on each side of the observed focus, as it was difficult to judge accurately with the eye on the ground glass, owing to the roughness of the latter and the feebleness of the light.

A picture, or rather many pictures, were now taken, and the one which by transmitted light most truthfully resembled a paper print from the small negative was reserved for further magnifying. This was effected absolutely in the same manner as the former: the negative being removed and the positive being placed in its stead, a further magnifying gave a large-sized negative.

Although this process seems very simple, it is impossible to estimate the difficulties, unless by an actual repetition of the experiment, which I had to overcome before arriving at the beautiful result which I have the honour to lay before the Society. The double copying had a tendency to slightly exaggerate the effect of light and shade, and this could only be obviated by exposing the plates for such a time, that with the feeble light at my command it was verging on decomposition; particles of dust, too, seemed most pertinaciously to fix themselves on the prominent mountains, giving rise to craters where none should be; and even my finished pictures are not per

fectly free from these faults, although each negative is the representative of a month's work and upwards of a hundred failures.

I doubt if much better photographs of our satellite can be taken by the way I have pursued. The future of lunar photography lies in another direction: the image must not be received on a sensitive plate and this copy submitted to an after process of magnifying. Defects quite imperceptible to the naked eye on the small negatives, are expanded into great blotches when magnified. In fact, upwards of a dozen seemingly equally good negatives with which I started, have, with but one or two exceptions, shown spots when enlarged.

The magnifying must be conducted simultaneously with the photographing, either by having the eyepiece on the telescope, or better still, by having a proper arrangement of lenses to throw a magnified moon image at once on the collodion. The difficulty of want of light could not be any objection, as supposing the enlarged image to be equal to those which I have now taken, that would be an increase of area of about twenty times, consequently 20 × 6 seconds, or 2 minutes, would represent the average time of exposure; a period which, even were it prolonged four or five times, would not then be too severe a tax upon a steady and skilful hand and eye.

Description of the Photographic Process.

The glass employed for taking the original negative of the moon, was that known as "extra white colour patent plate," that for the intermediate positives and large negatives was ordinary patent plate. Cleaning the surface, which is an operation of especial importance, was effected in the following manner.

The glasses were dipped into and then well rubbed over with a hot solution of caustic potassa; then, after washing with water, they were transferred to hot nitric acid (one part strong acid to three of water), where they were allowed to remain for about half an hour.

A piece of soft wash-leather was plentifully rinsed, first in a warm dilute solution of carbonate of soda, afterwards in clean water, and then well wrung until all the superfluous water was squeezed out. The glass plates were taken from the nitric acid and rinsed in abundance of clean water, and then rubbed well on every part with the damp leather. This removed most of the superficial moisture; and the final drying was effected by means of another piece of wash

leather, prepared the same as before, but allowed to become perfectly dry.

Just previous to using, the plates, held in a pneumatic plate-holder, had the last polish given to them by briskly rubbing with a warm piece of fine diaper (which had also been previously washed in soda and water, and then well rinsed and dried) until the moisture condensed from the breath evaporated evenly and uniformly, especially guarding against the slightest contact between the surface of the glass and the fingers.

The plate was now held with its clean side downward until the collodion was about to be poured on, and every particle of dust (which was easily seen by bringing the source of light, the under surface of the plate and the eye, nearly in the same line) was gently wiped off by passing a warm piece of fine cambric lightly across.

Care was also taken to have the atmosphere of the room as free as possible from floating particles, and the dried collodion usually adhering to the neck of the bottle was scrupulously removed.

The collodion was poured on and the plate rendered sensitive in the usual way. As the temperature both of the equatorial and operating rooms was seldom far from the freezing-point, the great diminution of sensitiveness, which that circumstance would have occasioned, was obviated by having the nitrate of silver bath and developing solution warmed to about 30° C., and also by slightly warming the plates before using. The source of light was a fishtail gas burner in the outer room, and shining close to the orange glass window of the dark room.

The soluble paper for the collodion was prepared in the following manner :-a mixture was made of

When the temperature of the mixture had cooled down to 50° C., one sheet of Swedish filtering-paper, torn up into small pieces, was completely immersed in the mixture, and allowed to remain therein for about half an hour. It was then thrown into a large pail of water, and the paper removed and placed on a sieve under a running tap for a quarter of an hour; after washing in very dilute solution of

ammonia and then in plenty of water, the paper was pressed between the folds of a cloth, and then allowed to dry spontaneously in the

air.

The collodion was made with

Ether, sp. gr. 725 (previously freed from acid by rectification from dry caustic potassa) ...... Absolute alcohol

Soluble paper (dried at 100° C.)

Iodide of cadmium (pure)........

The alcohol and ether were mixed together, and then the paper and iodide of cadmium were added: they dissolved in a few minutes with a little shaking. As soon as the solution was complete, it was allowed to stand for twenty-four hours, and then half of the clear supernatant fluid was decanted carefully into a clean well-stoppered bottle for use. I believe that collodion prepared in this way will remain uniform from one year's end to another.

The nitrate of silver bath was made by dissolving 1 ounce of crystallized nitrate of silver, perfectly pure and neutral, in 2 ounces of water, then, with constant stirring, adding a solution of 4 grains of iodide of cadmium in 1 ounce of water, and a quarter of an ounce of the above iodized collodion, and water to make up the volume to 10 ounces. This was allowed to stand for a few hours at a temperature of about 25° C., and then filtered from the undissolved iodide of silver and precipitated paper. A glass bath was used in preference to gutta percha, and, as above stated, it was heated to 30° C. when used.

The developing solution consisted of

Pure pyrogallic acid

Crystallized citric acid.

Water...

Alcohol.

8 grains. 16

This developing solution is very slow in its action, 15 to 20 minutes being frequently required, but it ultimately produces negatives of such vigour and freedom from stains, that I much prefer it to the usual formula.

The fixing solution employed was the ordinary nearly saturated. solution of hyposulphite of soda. After its employment the pictures