Astronomy 603 



observations with any sextant may be made exceedingly 

 accurate by careful handling, and by the application of the 

 corrections named above, and thus to create among ob- 

 servers and instrument makers a higher standard of work 

 and a consequent improvement in processes of manufacture. 



CONSTRUCTION OF A SILVERED-GLASS TELESCOPE 



Doctor Henry Draper published in 1864, through the me- 

 dium of the Smithsonian Institution, a description of his con- 

 struction of a telescope that has become a text-book for those 

 engaged in the making of silver-on-glass reflectors.^ 



The reflector has in some special researches, as in photog- 

 raphy and in some parts of celestial spectroscopy, a distinct 

 advantage over the refractor ; and as the aperture increases, 

 the difference in cost between a reflector and a refractor of the 

 same size is very marked. 



There is somewhere a limit to the size of a refractor due to 

 the fact that the incident light increases only as the square 

 of the aperture, while the absorption of the light in passing 

 through the glass increases in a far higher ratio. The ratio 

 of focus to aperture in large refractors must be relatively 

 large (in the Lick refractor it is 19 to i). On the other 

 hand, large reflectors can be built of relativel)- short focus (in 

 the Crossley reflector of the Lick Observatory the ratio of 

 focus to aperture is about 6 to i), and they can be constructed 

 at small cost, and mounted, — since they are comparatively 

 short, — in relatively small domes. They are very sensitive to 

 changes of temperature and to mechanical flexures, and for 

 these and other reasons lar^e reflectors are often inferior in 

 definition to refractors of equal aperture. But where, as 



1 " On the Construction of a Silvered-Glass lished in 1S64, and is the third paper in Vol- 

 Telescope, 15^ Inches in Aperture, and Its ume xiv of the "Smithsonian Contributions 

 Use in Celestial Photography." It was pub- to Knowledge." 



