October 19, 1900.] 



SCIENCE. 



603 



ou what lines should the telescope be built ? 

 I will endeavor to answer this. All the 

 work that is usually done by an astronom- 

 ical telescope, excepting very long-continued 

 observations, can be equally well done by 

 the fixed telescope. But there are some 

 special lines for which this form of research 

 is admirably suited, such as photographs of 

 the moon, which would be possible with a 

 reflecting mirror of, say, 200 feet focal 

 length, giving an image of some 2 feet di- 

 ameter in a primary focus, or a larger image 

 might be obtained either by a longer focus 

 mirror or by a combination. It might even 

 be worth while to build a special ccelostat 

 for lunar photography, provided with an 

 adjustment to the polar axis and a method 

 of regulating the rate of clock to correct the 

 irregular motion of the moon, and thus ob- 

 tain absolutely fixed images on the photo- 

 graphic plate. 



The advantage of large primary images 

 in photography is now fully recognized. For 

 all other kinds of astronomical photography 

 a fixed telescope is admirably adapted ; and 

 so with all spectroscopic investigations, a 

 little consideration will show that the con- 

 ditions under which these investigations 

 can be pursued are almost ideal. As to 

 the actual form such a construction would 

 take ; we can easily imagine it. The large 

 mirror mounted as a ccelostat in the center ; 

 circular tracts around this center, on which 

 a fan-shaped house can be traveled round 

 to any azimuth, containing all the necessary 

 apparatus for utilizing the light from the 

 large plane mirror, so as to be easily moved 

 round to the required position in azimuth 

 for observation. In place of a fan-shaped 

 house movable round the plane mirror, a 

 permanent house might encircle the greater 

 portion round the mirror, and in this house 

 the telescope or whatever optical combina- 

 tion is used might be arranged on an open 

 framework, supported on similar rails, so 

 as to run round to any azimuth required. 



The simplicity of the arrangement and the 

 enormous saving in cost would allow in 

 any well-equipped observatory the use of a 

 special instrument for special work. The 

 French telescope has a mirror about 6 feet 

 in diameter and a lens of about 4 feet. 

 This is a great step in advance over the 

 Yerkes telescope, and it may be some time 

 before the glass for a lens greater than 50 

 inches diameter will be made, as the difii- 

 culty in making optical glass is undoubtedly 

 very great. But with the plane mirror 

 there will be no such difiBculty, as 6 feet 

 has already been made ; and so with a con- 

 cave mirror there would be little difficulty 

 in beginning with 6 feet or 7 feet. The way 

 in which the mirror would be used, always 

 hanging in a band, is the most favorable 

 condition for good work, and the absence of 

 motion during the observation, except of 

 course, that of the plane mirror (which could 

 be given by floating the polar axis and suit- 

 able mechanical arrangements, a motion of 

 almost perfect regularity). 



One extremely important thing in using 

 silver or glass mirrors is the matter of re- 

 silvering from time to time. Up to quite 

 recently the silvering of my 5-foot mirror 

 was a long, uncertain, and expensive proc- 

 ess. Now we have a method of silvering 

 mirrors that is certain, quick and cheap. 

 This takes away the one great disability 

 from the silver or glass reflecting telescope, 

 as the surface of silver can now be renewed 

 with greater ease and in less time than the 

 lenses of a large refracting telescope could 

 be taken out and cleaned. It may be that 

 we shall revert to speculum metal for our 

 mirrors, or use some other deposited metal 

 on glass ; but even as it is we have the sil- 

 vered glass reflector, which at once allows 

 an enormous advance in power. To do jus- 

 tice to any large telescope it should be 

 erected in a position, as regards climate, 

 where the conditions are as favorable as 

 possible. 



