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this greatest instrument of the age are flector, with silvered glass mirror and concisely and comprehensively described with a well-designed mounting, may in the following letter from Prof. Ritchey justly be regarded as the most modern to the donor which was published in the and efficient type of telescope. Los Angeles Evening News, Sept. 15, It should be remembered that a reflect1906:

ing telescope gives superb views of celesSolar Observatory Office, Pasadena, Cal. tial objects visually, also, but it is in pho

July 27, 1906. tography that it is incomparably efficient; Mr. John D. Hooker, Los Angeles. and it is here that results of the greatest Dear Mr. Hooker:

scientific value will be obtained. I have your letter of July 26, in regard First: An eight-foot reflector, which

should have a principal focal length of fortyeight feet, would give revolutionary results in the photography of the nebulae. This is the subject in which I am personally most interested. My photographs of nebulae made at the Yerkes Observatory were obtained with the two-foot reflector, which had a focal length of only ninety-three inches. The eight-foot reflector would have a focal length of forty-eight feet, or 576 inches, that is, six and one-fifth times as great as that of the two-foot reflector; the scale or size of the photographs would be in the same proportion; and with the smooth motion of the telescope

given by mescury flota. Five-Foot MIRROR OF NEW 60-INCH REFLECTOR.

tion, together with the

fine atmospheric definito the field of usefulness of an eight-foot tion and transparency to be had on reflector. The chief uses to which we Mount Wilson, we could certainly expect would put such an instrument are briefly a proportional gain in the minuteness of described below. You will note that all detail and structure shown in these neof these, except the measurement of the bulae. heat of stars, relate to photography. The The great majority of the nebulae are reflecting telescope is especially efficient small and faint—too small for such inin photography, because of its perfect struments as the two-foot reflector at the achromatism and also. because of its Yerkes Observatory, or even the Crossgreat speed, since the percentage of light ley reflector at the Lick Observatory. In lost by reflection at the silver film is much fact, there are probably not more than smaller than that lost in passing through forty or fifty nebulae which are sufficientthe lenses of a large refractor. By far ly large for the best results with instruthe most promising lines of advance in ments of the scale of those just named, astronomical investigations are in the di- while the eight foot reflector would give rection of photography; hence a great re- us photographs of tens of thousands of

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these small objects, on a sufficiently position and the approximate temperalarge scale for the most refined measure- ture and pressure in their atmosphere can ment. One great purpose of this work be determined, as well as their motions is to detect changes of form, that is, evi- toward or away from the earth. The dences of development, in nebulae. It is light of some of the bright stars could plain that the larger the scale of the also be analyzed as completely as that of photographs and the finer the detail the sun has been. More important than shown, the greater is the chance of de- this, however, is the fact that it would at tecting changes of form within a reason- once become possible to trace out the evoable time.

lution of stars, and their development Second: By the addition of a convex from nebulae, with far greater certainty magnifying mirror, twenty-eight inches than at present. in diameter, used in conjunction with F ourth: The only measurements we the eight-foot mirror, the focal length of have of the heat of the stars were made the eight-foot reflector could be increased with the two-foot mirror at the Yerkes to four by forty-eight feet-192 feet, or Observatory. They showed that Arctueven to six by forty-eight feet—288 feet. rus gives us about as much heat as a Used in this way this telescope would be candle at a distance of six miles. The suitable for photographing the kinds of eight-foot reflector would give sixteen objects which I photographed with the times as much heat as the two-foot, and forty-inch 'refractor, with the yellow color permit the accurate measurement of a screen, namely, bright and small objects, great number of stars, whereas only two such as the globular star clusters, the stars could be measured in the earlier planetary nebulae, the moon, and the work. planets; but it would be incomparably I hope the above will give you the inmore efficient for this work than the for- formation which you desire. If you wish ty-inch, for the following reasons: (1) for further details, however, I shall be the scale would be greater in the propor- most happy to furnish them. tion of 288 to 63 (sixty-three feet is the

Very sincerely yours, focal length of the forty-inch); (2) the

G. W. RITCHEY, reflector would give greater speed on ac- Superintendent of Instrument Construccount of the small loss of light.

tion. I feel certain these photographs would A brief account of the various proprove as revolutionary as those of the cesses which will be emploved in the faint nebulae obtained at the primary fo- manufacture of a hundred-inch mirror cus. To illustrate: the image of the will enable one to form some idea of the moon on the original negatives, as photo- obstacles which must be overcome, and graphed with the forty-inch refractor at to appreciate the infinite pains and study the Yerkes Observatory, is seven inches essential to the successful completion of in diameter; the image given with the the work. To construct a lens weighing eight-foot reflector, with equivalent fo- four and one-half times more than any in cal length of 288 feet, would be thirty- existence it will be necessary to make two and one-fifth inches in diameter. The many changes in the accessories of the same ratio of increase holds for the glass works and instrument shops. planets, star clusters, etc. .

At the ancient glass works of St. GoThird: In the photography of the bain a specially devised crucible of firespectra of stars and nebulae, the eight- proof clay will be heated gradually for foot reflector would enormously surpass many days and placed in a melting-oven all existing instruments. In this work it until white-hot. The materials to comis simply a question of collecting the pose the glass will be slowly introduced greatest possible amount of light into the through an opening in the top of the star image. As the eight-foot would give oven, the operation consuming possibly seven times as bright a star image as the forty-eight hours. In case there is no acCrossley reflector, an immense number of cident to the melting-pot or oven, which stars, now inaccessible, would be brought are frequently cracked by the tremendwithin range. As soon as their spectra ous heat, the impurities are skimmed off can be photographed, their chemical com- as they rise to the surface, and the whole

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