PHOTOGRAPHING THE STARS. 305 



The long exposures necessary for celestial photography 

 have introduced a new class of requirements into the con- 

 struction of astronomical instruments. The questions here 

 involved are of much practical importance, and are ex- 

 citing a good deal of discussion at present. 



There are, as is well known, two different classes of 

 astronomical instruments namely, the reflectors and the 

 refractors ; and it is still a matter of debate as to which 

 class of instrument is the more suitable for the purposes 

 cf celestial photography. 



In the reflector the rays from the star fall on the bril- 

 liant surface of a mirror carefully wrought into a special 

 form. Formerly mirrors were made of speculum metal, 

 consisting of two parts of copper to one of tin. This 

 material was difficult to cast and tedious to shape. Its 

 great weight was also a drawback, while the reflecting 

 power, though very considerable, was still short of that 

 possessed by silver. At present most of the reflectors are 

 made of glass, and, after being accurately ground and 

 polished to the true form, are chemically coated with silver. 



The mirror, when used for celestial photography, is at 

 the lower end of a tube, and the rays falling upon it from 

 the star travel again up the tube to a focus on the plate, 

 which is exposed with its face towards the mirror at the 

 upper end. The plate is supported by slight arms from 

 the side of the tube, and it offers of course an impene- 

 trable obstacle to some of the rays from the star, and so 

 far diminishes the effective size of the mirror. As how- 

 ever the diameter of the plate will not be more than 

 perhaps one-fifth that of the mirror, it follows that only 

 about four per cent, of light is lost by this cause. The 

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