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A^A TURE 



\_AugMst 29, 1889 



apiece to any astronomer desiring to discuss them. Or 

 an edition of paper photo-engravings could be issued. 

 A complete set of about 1200 plates could probably be 

 furnished for about $200. This would be a very small 

 sum compared with the cost of a telescope which would 

 show far less. The cost of the lens should not exceed 

 $20,000 ; of the prism, ^5000 ; of the mounting, ^5000 ; 

 of the building, $5000 : total, $35,000. A large additional 

 sum might be spent upon a building, but the experience 

 '■3f modern astronomers has shown that the best results 

 are obtained by mounting each instrument in a small 

 detached building which will readily assume the tem- 

 perature of the air. It is more difficult to estimate the 

 -current expenses. The photographic plates alone would 

 tost $1500 annually. The total annual expenses would 

 not be less than $5000, not including superintendence, 

 reductions and discussion of the observations, and publi- 

 cations. An outlay of ^50,000 would probably complete 

 the instrument, and secure photographs of the entire 

 northern sky. If the anticipated results are attained, 

 this should be followed by an endowment of .^100,000, 

 which would keep the instrument permanently at 

 work. 



The faintest stars photographed with the 8-inch 

 telescope in Cambridge are invisible with the 15-inch 

 telescope. The inner satellites of Uranus have been 

 photographed at Cambridge with a 13-inch lens, although 

 they are among the most difficult test objects known. 

 We may therefore expect that stars too faint to be 

 ■detected with any other instrument might be photo- 

 graphed with a 24-inch lens. We thus see that any 

 person could obtain at a comparatively small cost a map 

 of a portion of the sky showing stars too faint for him to 

 observe in any other way. Many investigations might 

 be carried on by means of these plates ; for instance, a 

 search for double stars, for nebulae, for asteroids, for 

 variable stars by comparison of plates of the same region 

 taken on different nights, for stars having large proper 

 motion when we have plates repeated after a considerable 

 interval of time. In all these cases the plate furnishes 

 an accurate measure of the object discovered, which is 

 often wanting in the first observation of an object by the 

 eye. Studies of the distribution of the stars can now 

 scarcely be undertaken in any way except by photo- 

 graphy. The stellar spectra might lead to the discovery 

 of planetary nebulae, variable stars, bright-line spectra, 

 and other interesting objects. The number of stars 

 shown on the charts would be so vast that it would 

 probably be impossible to catalogue them all. In some 

 cases it is estimated that twenty or thirty thousand stars 

 have been photographed upon a single plate. A syste- 

 matic examination of all the plates for the detection of 

 objects of interest would itself be a laborious undertaking. 

 Such work could, however, be done at any place and time, 

 and therefore under the most favourable conditions as re- 

 gards expense. One great advantage of the work would be, 

 not only that we could discover certain objects of a given 

 kind, as a particular class of double stars, but that we 

 could make our catalogue complete, and be sure that no 

 other objects of this kind existed, which in visual work is 

 an extremely difficult matter. 



There is always danger of failure in the construction of 

 a new instrument. This danger is diminished in the 

 present case, since an instrument of one-third the size is 

 already in successful operation. Moreover, the front 

 portion of the proposed double lens should form a jjood 

 photographic objective, and might be made of the rever- 

 sible form, which may be used either for visual or photo- 

 graphic purposes. The 13-inch lens referred to above is 

 of this form, and proves that such an instrument is 

 entirely practicable. We should thus be able to use the 

 telescope in three ways : for visual purposes, as a telescope 

 having an aperture of 24 inches, and a focal length of 

 17 feet ; as a single photographic lens of the same dimen- 



sions ; and as a photographic doublet covering a large 

 field, and having a focal length of 1 1 feet. The prism 

 could be used with each of these, making really six in- 

 struments in one. The short focal length would greatly 

 diminish the cost of the mounting, and of the dome re- 

 quired to contain the instrument. The difficulties from 

 exposure to violent winds and storms would also be much 

 less than in an instrument of the usual form. 



Should the proposed plan be carried out successfully, 

 a contribution to astronomy would be made of continually 

 increasing value, since each year would increase the 

 facility with which slow changes in the stars might be 

 detected by the comparison of later photographs with 

 those first made. 



In connection with the second circular we may remind 

 our readers that we lately referred to the fact that the 

 Astronomical Observatory of Harvard College has re- 

 ceived from Miss C. W. Bruce, of New York, a gift of fifty 

 thousand dollars to be applied " to the construction of a 

 photographic telescope having an objective of about 24 

 inches aperture, with a focal length of about 1 1 feet, and 

 of the character described by the Director of the Obser- 

 vatory in his circular of November last ; also to secure 

 its use under favourable climatic conditions in such a 

 way as in his judgment will best advance astronomical 

 science." 'ihis, then, is a very concrete reply to the first 

 circular. 



The second circular contains full details of the in- 

 strument, which will differ from other large telescopes 

 in the construction of its object-glass, which will be 

 a compound lens of the form used by photographers, 

 and known as the portrait lens. The focal length of such 

 a lens is very small compared with its diameter, and 

 much fainter stars can be photographed in coasequence. 

 The advantage is even greater in photographing nebulas 

 or other faint surfaces. Moreover, this form of lens will 

 enable each photographic plate to cover an area several 

 times as great as that which is covered by an instrument 

 of the usual form. The time required to photograph the 

 entire sky is reduced in the same proportion. A tele- 

 scope of the proposed form, having an aperture of 8 

 inches, has been in constant use in Cambridge for the 

 last four years, and is now in Peru, photographing the 

 southern stars. It has proved useful for a great variety 

 of researches. Stars have been photographed with it, 

 too faint to be visible in the 15-inch refractor of the Ob- 

 servatory. Its short focal length enables it to photograph 

 as faint stars as any which can be taken with an excellent 

 photographic telescope having an aperture of 13 inches. 

 The 8-inch telescope will photograph stars about two 

 magnitudes fainter than can be taken with a similar in- 

 strument having an aperture of 4 inches. A corresponding 

 advantage is anticipated from the increase of the aperture 

 to 24 inches. Each photograph will be 13 inches on a 

 side, and will cover a portion of the sky 5' square, on a 

 scale of 1' to a millimetre. The dimensions will be the 

 same as those of the standard charts of Chacornac and 

 Peters. The entire sky would be depicted upon about 

 two thousand such charts. 



In an article in the Observatory for August the action 

 taken by Prof. Pickering in making the appeal for a 

 money grant to carry out the new map is distinctly 

 challenged on the grounds— (i) that in the appeal there 

 was no statement made that very nearly similar work 

 was about to be commenced by all the civilized nations of 

 the world after most careful consideration of the whole 

 question by the strongest gathering of astronomers that 

 has ever been seen ; (2) that Prof. Pickering suggested 

 to the Conference the use of a telescope similar to the 

 one he has induced Miss Bruce to supply ; and (3) that 

 there are conditions of construction and execution of 

 such difficulty that Prof. Pickering's plan may fail 

 altogether. 



