NEW YORK, Feb. 2, 1864.

THE first photographs of the moon were taken in 1840 by my father, Professor John W. Draper, M.D., who published notices of them in his quarto work, On the Forces that Organize Plants,' and in the 'Philosophical Magazine.' The specimens were about an inch in diameter, and were presented to the Lyceum of Natural History of New York. They were made by means of a lens of five inches aperture, furnished with an eye-piece to increase the magnifying power, and mounted on a polar axis driven by a clock. At that time it was generally supposed that the moon's light contained no actinic rays, and was entirely without effect on the sensitive silver compounds used in daguerreotyping.

In 1850, Mr. Bond made use of the Cambridge (Massachusetts) refractor of 15 inches' aperture, to produce daguerreotype impressions of our satellite, the sensitive plate being placed at the focus of the objectglass, without the intervention of an eyepiece. Pictures two inches in diameter were thus produced, and, subsequently, some of the same size were made on glass, and mounted stereoscopically. Mr. Bond also made a series of experiments to determine whether photography could be advantageously applied to the measurement of double stars, and concluded that the results were as reliable as those derived from the micrometer.*

Soon after, Mr. Warren De La Rue, of Cranford, near London, undertook by the aid of a 13-inch speculum, ground and polished by himself, to procure a series of photographs of the moon and other celestial objects. The excellent re

*Astron. Nach,' No. 1129.

sults that he has obtained, together with those of Professor Phillips, Mr. Hartnup, Mr. Crookes, Father Secchi, and other physicists, are doubtless familiar to all scientific men, having been published in the form of a report to the British Association in 1859. No detailed description of them is necessary, therefore, in this place.

In 1857 Mr. Lewis M. Rutherfurd, of New York, erected an equatorial refractor of 11 inches' aperture, the object-glass of which he had himself corrected, and has taken a large number of lunar photographs with it. They have generally borne to be magnified to five inches, and he is now engaged in perfecting a correcting lens that will allow still greater enlargement to be used.

The moon, as seen by the naked eye, is about one-tenth of an inch in diameter, although persons generally estimate it at 10 inches. That the first statement is true is easily proved either by taking a photograph with a lens of 10 inches' focal length, or more convincingly by holding up between the moon and the eye a little disc one-tenth of an inch across, at the nearest distance of distinct vision (10 inches). A picture of the moon of the size commonly attributed to her requires to be made under a power of 100 times.

In 1857 I visited Lord Rosse's great reflecting telescopes at Parsonstown, and had an opportunity of not only seeing the grinding and polishing operation by which they were produced, but also of observing some stars through the six-foot instrument. On returning home in 1858 it was determined to construct a large instrument by similar means, and devote it especially to celestial photography. The speculum was of

15 inches' aperture, and 12 feet focal length. Subsequently, however, this metal mirror was abandoned, and silvered glass, as suggested by M. Foucault, substituted. This latter, according to Steinheil's experiments, reflects more than 90 per cent. of the light falling upon it, while speculum metal only returns 63 per cent. A detailed account of this instrument, amply illustrated, is now being published by the Smithsonian Institution at Washington, and therefore only a general idea of its peculiarities will be given.

As the telescope was intended especially for photography, the following general principles were adopted. 1st. A reflector was, of course, preferred to an achromatic object-glass, because all the rays falling upon it are reflected to the same focal plane, and there is not, as in the latter, one focus for distinct vision, and another for the photographically actinic rays, an inch distant perhaps. In the reflector a sensitive plate put where the image is seen to be most sharply defined, will be sure to give a good result. In the achromatic, on the contrary, the sensitive plate must be placed in a position which can only be found by tedious trials. 2nd. Silvered glass was used instead of speculum metal, because it is lighter and more highly reflecting. Besides, if a reddish or yellowish film should accumulate on it-an accident liable to occur to either kind of reflector and seriously diminishing the photographic powerit can either be repolished with a piece of buckskin-an operation obviously impossible in the case of a speculum metal-or the silver can be dissolved off with nitric acid, and a new film deposited on the glass concave. The glass which has been made accurately parabolic before the first silvering, is not changed in figure, the silver being only deposited in a layer 2000 of an inch thick, and consequently, if carefully prepared, copying the glass below so closely that no error larger than a small fraction of that amount is possible. As the glass only serves as a basis or mould for the thin

sheet of silver, and is not penetrated by the light, its quality is a matter of but little moment, that which is used for skylights or light-openings in floors answering perfectly. 3rd. A mounting, presenting the greatest degree of steadiness possible was necessary. For this purpose the telescope was supported at both ends, the lower one resting in a loop of wire rope. 4th. Instead of driving the whole mass of the instrument by clockwork acting upon a polar axis, and thus being forced to move a weight of at least half-a-ton-the usual system in equatorials only the sensitive plate and its frame, weighing an ounce, were caused to follow the moon or other object, the mass of the apparatus remaining perfectly at rest. This idea is due to Lord Rosse. 5th. Instead of using a clock with wheelwork for a prime mover, a clepsydra was substituted. This consists of a heavy weight supported by the rod of a piston, which fits into a cylinder filled with water. At the bottom of the cylinder a stopcock permits the water to flow out at a variable speed, depending on the amount of opening. The sensitive plate can thus easily be caused to coincide in rate with the moving object, and yet by a motion free from irregularity and tremor.

The value of a silver reflector turns, of course, entirely upon the perfection of the glass concave on which the metallic film is to be de

posited. This must be of a parabolic figure, so that spherical aberration may be completely corrected. A person is, however, content to take the utmost pains to produce it, because, once attained, the figure cannot be lost except by fracture, and the value does not diminish with time as in the case of a speculum. It never requires re-polishing. The best method of grinding and polishing the glass is by means of an apparatus that I have called a "Local-correcting Machine," by which all the parts of the surface can be attacked in succession and reduced to the desired curvature,

and yet at the same time a uniform curve and absence of local irregularities secured. I have spent five years in the investigation of this subject, and have polished more than 100 mirrors of from 19 inches to one-fourth of an inch in diameter, on seven different machines built at various times. The quality of those I have at present is indicated by the fact that they will show Debillisima to be quintuple, and will render the close companion of Sirius, discovered by Alvan Clark's magnificent 18-inch refractor, visible.

The Observatory at Hastingsupon-Hudson, near New York, lat. 40° 59′ 25′′ N., long. 73° 52′ 25" W. of Greenwich, is upon the summit

of a hill 225 feet above low-water mark. It is 20 feet square, with a wing 9 x 10 for a photographic laboratory. As the telescope is a Newtonian, with the mounting so contrived as to have the eyepiece stationary at all altitudes, a plan originally suggested by Miss Herschel, there are peculiar facilities offered for easy access to the eyepiece, or place of the sensitive plate. The interior height of the Observatory, 22 feet, is divided into two stories, around the upper of which an observer's chair runs to follow the telescope. The dome turns upon a pivot at its centre, instead of on rollers or cannon-balls around the edge, and is moved consequently with but slight exertion.

In the woodcut a is the telescope tube, b one of the trunnions on friction rollers perforated for the eyepiece, c one of the counterpoise levers, having a weight at the upper end and being attached to an axle d at the lower end; eee" is a wirerope going from the counterpoise to the lower end of the telescope; ff another wire - rope which passes round a small drum connected with the winch g, and gives the observer standing on the observer's platform i the power of moving the telescope in altitude; k the stairs going to the photographic room, the gallery that divides the Observatory into two stories, m the azimuth axis resting on the solid rock, and sustained at its upper end by the three lateral beams, nn' (two only are shown). They also rest in cavities in the rock. The dome is seen in section at oo', the dome-opening and shutter at p, the dome-arch at qq. The dome-raising lever r, with the fulcrum at s, is shown as it appears when the dome is prepared for revolving, the axis t carrying the whole weight. The part of the lever below the detent u can be bent up out of the way, and held by a loop.

Since the telescope has been completed, and furnished with two parabolic mirrors of 15 inches aperture, and 150 inches focal length, and one Herschelian mirror (that is, a concave of such figure that it can only bring oblique pencils to a focus free from aberration), Celestial Photography has been continually prosecuted. About 1,500 lunar negatives have been taken. Old experience obtained from portrait and microscopic photography has proved to be of great service. At first the well-known processes were used, but it was soon found that something more refined was needed, where the pictures are to be submitted to magnifying powers of perhaps 25 times. Defects in collodion negatives that would, under ordinary circumstances, pass unnoticed, assume such prominence as greatly to diminish the beauty of the results.

These defects, pin-holes, coarse granular appearance of the reduced silver, and other markings, were found to arise principally from the presence of nitrate of silver on the sensitive plate. It was ascertained that by washing the plate thoroughly before exposure they disappeared, or were very much ameliorated, and without any reduction in sensitiveness. But for this washing operation pure water is needed, and hence the roof of the buildings was painted with a ground mineral compound that hardens to a stony consistence, and the water falling upon it was preserved in a leaden tank, which from long use for other purposes had become thickly coated with insoluble salts of lead, sulphates, &c. Whenever an inch of rain falls, a ton of water is collected, and the tank may be filled about 32 times in a year.

The negatives produced at the focus of the reflector are on an average 1 inches in diameter. Many that have been made will bear to be enlarged to 2 feet, and one was taken September 3, 1863, at 4.30 A.M., which has been increased to 3 feet in diameter, the total magnifying power used being about 380. In this photograph the moon may be said to be shown on a scale of 60 miles to the inch.

In the process of enlarging I have introduced one very important, novelty. Instead of employing an achromatic combination of lenses arranged as a solar camera, a concave mirror is used. It entirely gets rid of the difficulty of chromatic aberration, which is, as all photographers know, one of the most serious obstacles to success, and, in addition, the magnified image lies in one plane, or there is what is termed a flat field. Every little detail of the original negative is perfectly reproduced, and a 3-foot image is as sharp in one part as in another. The effect of portraits reproduced of life-size is very striking, and the resemblance to the individual singularly increased. In magnifying these lunar negatives, a

mirror of 8 inches' aperture and 11 inches' focal length is used. At first, when it was intended to employ diffused daylight and the whole aperture, the figure was made elliptical, with a distance of 8 feet between the conjugate foci; but subsequently, when the advantages of sunlight were understood, the surface was reduced by a diaphragm to 1 inches in diameter, and a part of the mirror as nearly perfect as a mirror can be made at present was selected. Success in enlargement becomes with this contrivance a certainty.

The "enlarger" is also equally valuable in copying by contact. When a small negative is enlarged and photographed, what is termed a positive results. If such a positive is used to make prints on paper, the lights and shades are inverted, and that which is white is shown black. It is necessary then to turn the original negative into a positive, so that when magnified a negative may result suitable for printing positive proofs on paper. This is done usually by a process called reversing, in which a sensitive plate is placed behind the original negative, and the two exposed to the light. Wherever the negative is transparent the plate behind is stained by the light, and where opaque, it is protected. But unless the plate behind is so close as to make the chances of scratching the negative very great, the positive produced is much inferior in distinctness, because the diffused light of day finds its way through in many directions. If, however, the negative and sensitive plate are placed in the beam of sunlight coming from the enlarger, the rays pass through only in one direction, and the reverse or positive is as sharp as the original negative.

Celestial photography is as yet only in its infancy. The results to which it has given origin, although excellent in many respects, have imperfections. But it seems probable that these may be overcome in the future, partly by means now within

reach, and partly by others which may be discovered at any moment. In looking at a 3-foot photograph from such a distance that the eye can embrace it all at one glance, the general effect is certainly very fine, and superior to observation through the telescope with a similar power. The moon appears as it would if viewed from a stand-point 600 miles from its surface. Ranges of mountains, as the Apennines, seem as if projected out from the general level, while the great craters, such as Plato, Theophilus, and Clavius, are deeply excavated below. Grooves

of vast extent, like those diverging from Tycho, and faults such as that running past Kant and Catharina on the one side, and Tacitus on the other towards Lindenau, still further break up the surface. The wellknown seas and bright portions, so distinct to the naked eye, are lost in the multiplicity of the details into which they are resolved.

But coming more closely to the picture, and examining with a critical eye, it is apparent that, although the general effect is the same as would be perceived by looking at the moon itself, yet some of the minute details seen in the telescope with a high power are absent.

The reasons which lie at the bottom of this difficulty are connected to a certain degree with the photographic processes employed, but also to not a little extent with the condition of the air. The quality of the instrumental means used is, of course, of primary importance. A good photograph cannot be taken with an inferior telescope and clock.

The obstacles arising from photography result from the fact that the dark parts of the picture are not formed by a continuous sheet of material, but by an aggregation of granules which, though invisible to the unassisted eye, are seen when a high-enough magnifying power is employed. Their degree of visibility turns on the system of development used for bringing out the latent image on the sensitive plate. A picture injudiciously forced with