126 CARNEGIE INSTITUTION 



stars. What you want is an instrument that will collect the largest 

 possible amount of light from a star within the jaws of a spectro- 

 scope slit. 



Not only does a reflector provide this on a larger scale than it is 

 possible to attain in a refractor, but it unites the rays of every re- 

 frangibility in one focus. 



For spectroscopic work a Cassgrain reflector seems the best form 

 to adopt, as the cone of rays, with its smaller angle of convergence 

 to focus, permits use of a comparatively long collimator. The posi- 

 tion of the spectroscope is also convenient, because, having regard 

 to the weight of the speculum, the radius of motion of a spectro- 

 scope near the speculum end must be much smaller than that of one 

 attached near the principal focus of the large mirror, and it is also 

 much more convenient of access in the former than in the latter 

 case. 



Of course, photographs of nebulae, &c, could be taken in the 

 focus of the principal mirror, but this is a less urgent need than the 

 spectroscopic researches. 



With the modern ball bearings and electric-motor motions I see 

 no difficulty in conveniently mounting a mirror of 6 or 8 feet 

 diameter. 



The erection of such an instrument in some very favorable posi- 

 tion is, I think, the next great step that should be taken. We are 

 in a position now, with refined and well studied apparatus, to at- 

 tack the determination of motions in the line of sight of all the 

 brighter stars ; but to get sufficient light to photograph the spectra 

 of the fainter stars under such dispersion as will furnish reliable 

 determination of motion in the line of sight requires a telescope of 

 greater light grasp than I fear we shall ever get from a refracting 

 telescope. For less money than the cost of a 40-inch refractor one 

 could mount a reflecting telescope of twice that aperture — i. e. , of 

 four times the light grasp — that would unite all the rays of light 

 from a star in one focus. 



I put this so far beyond all other demands of astrophysics that I 

 make no further suggestion, and I do feel that steps should be 

 taken to urge its fulfilment. 



I believe that Bloemfontein, in the Orange River Colony, would 

 be an ideal site for the erection of such an instrument. 



