J.OLY 2, 1894.] 



KNOWLEDGE. 



145 



-i^ 



-^OVfLEOQ 



AN ILLUSTRATED 



MAGAZINE OF SCIENCE 



^ 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: JULY S, 1894. 



contents; 



PAGE 



On the Mounting of Large Telescopes. By Prof. Geo. 



E. Hale... ' 145 



Insect Secretions.— III. By E. A. Butleb, B.A., B.Sc. ... U6 



Liquid Air. Bv .J. J. Stehabt, B.A.Cnntab., B.Se.Loiul. ... U9 



The Man-like Apes. By R. Lydekkeb, B.A.Cantab., F.R.S. l.jl 



The Defining Power of Insects' Eyes. Bv A. C, Rantard I-jO 



Notices of Books... I.-jS 



On the Changes of Form in Comets Tails. By A. C. 



R lyyARii 159 



Letters: — C. MiiiiiE Smitr; William Noble ; Edw. G. 



Gii.iiKET. M.D UiO 



The Venom of the Vipers. By C. A. Mitchell, B.A.Oxoa. Kil 

 Types of Floral Structure. Bv the Eer. A. S. WiLsox, 



M.A., B.S,- ■ 163 



Natures Protest against Change. BTVArOHAN' Cornish, 



M.Sr., F.C.S .'. 164 



The Face of the Sky for July. By Herbert Sadler, 



F.R.A.S. 166 



Chess Column. By C. D. Lococz, B.A.Oion 167 



ON THE MOUNTING OF LARGE TELESCOPES. 



By Prof. Geo. E. Hale. 



EVERYONE interested in the mounting of large 

 telescopes will see iu Sir Howard Grubb's paper, 

 in your ilay number, a most valuable contribution 

 to the literature of the subject. It seems likely 

 that we are soon to be brought face to face with 

 the problem of de^^sing suitable mountings for instruments 

 of great aperture, and the novel modification of Dr. 

 Common's flotation method, described in the article referred 

 to, contains features which commend it at once. At the 

 same time it is not without disadvantages, but whether or 

 not these are to be regarded as serious, experiment can 

 best decide. 



It is hardly to be doubted that Sk Howard is right in 

 looking to the reflector as the most promising means of 

 furnishing greatly increased light-grasping power. Such 

 able opticians as Brashear and Clark have told me that 

 they are ready to make objectives of as great aperture as 

 the optical glass furnished them will allow, and during a 

 recent visit to Jena I found Dr. Schott confident that he 

 could soon make discs of hitherto impossible size. Never- 

 theless, an aperture of sixty inches is the largest I have 

 ever heard of discussed for an objective, while Dr. Common 

 has at least two mirrors of this size, not to mention Lord 

 Rosse's great instrument. In default of glass, speculum 

 metal could be used, and I feel certain that Brashear would 

 undertake a mirror of ten feet aperture. 



It is apparently only too true that for many classes of 

 work the reflector is inferior to a refractor of the same or 

 even much smaller aperture, but we may at present con- 

 sider only such problems as require for their solution an 

 instrument of great light-grasping power. I quite agree 

 with Sir Howard that "the chief hope of progress in 

 astronomical research lies undoubtedly in the application 

 of photography," and the great telescope under con- 

 sideration should most certainly be capable of being used 

 for this purpose. But the possibility of photographing a 

 field of stars or uebuhv at the focus of the instrument, while 

 most eminently desirable, is not the only advantageous use 

 to which a large instrument could be put. Indeed, I 

 venture to believe that the employment of such a large 

 telescope for the photography of stellar spectra would be of 

 even greater value in the advancement of science than the 

 results to be obtained by ordinary photographing in the 

 principal focus ; for so fruitful is the study of the spectra 

 of the brighter stars with small telescopes, that a far more 

 abundant harvest may be expected when greatly increased 

 apertures bring the countless number of fainter stars within 

 reach. In the case of the brighter stars, the manifold 

 increase of dispersion made possible by the gain in light 

 would render still more accurate than at present the 

 measure of motions in the line of sight, and the spectro- 

 scopic method would become a hopeful means for the 

 determination of the solar parallax. But the importance 

 of stellar spectroscopy is too evident to require discussion, 

 and I will merely point out one practical reason for 

 designing the telescope with this end in view. If the 

 definition of the instrument from any cause should turn out 

 to be defective, unsuiting it for direct visual or photographic 

 work, it might still be of great service in spectroscopy, 

 especially with a spectroscope having a long collimator. 

 Thus, if the telescope be of a design suitable for spectro- 

 scopic work, its construction need hardly be looked upon 

 as an experiment, for the probability that it would not 

 suffice for this purpose is very small. 



If the desirability of constructing the telescope for 

 spectroscopic work be admitted, the focal length must 

 certainly be much longer than that chosen by Sir Howard 

 Grubb. With a spectroscope of given aperture, the bright- 

 ness of a star's spectrum is independent of the focal length 

 of the large telescope. If we choose a collimator of about 

 twenty-four inches focus, and give it an aperture of two 

 inches (more than this would hardly be advisable, on 

 account of the great size and weight of the prism-train), 

 we have a ratio of one-twelfth. With an aperture of ten 

 feet the great reflector would thus have a focal length of 

 one hundred and twenty feet. In any case I hardly think 

 a ratio greater than one-tenth would he advisable from a 

 spectroscopic standpoint. Sir Howard has chosen a ratio 

 of about one-fifth, giving a focal length of fifty feet for an 

 aperture of ten feet. DoubUng the length of the tube 

 would certainly render the problem of mounting the 

 instrument much more difticult, but this very increase 

 emphasizes the necessity of employing some such method 

 of supporting the tube as that devised by Sir Howard 

 Grubb. 



One of the greatest defects of all forms of telescope 

 mounting at present employed is the method of sup- 

 porting the tube at a single point, which is usually near 

 the centre. In small instruments the difficulty is not felt, 

 but as the focal length and weight of the objective increase, 

 the question becomes serious. In the case of the forty- 

 inch Yerkes telescope the objective will weigh about five 

 hundred pounds, and the solar spectroscope nearly half a 

 ton. These weights (with additional counterpoise at the 

 objective end) will be carried at the opposite ends of a 



