70 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



modifications, especially those for measuring purposes, before 

 we can follow the advances made by its aid. 



One common form will be gathered from Fig. 28, which re- 

 presents a small instrument with one prism. On the left we 

 have the collimator b, at the outer extremity of which is the fine 

 slit, the width of which can be regulated to a nicety by a micro- 

 meter screw. The spectrum is observed by the telescope /, 

 which is simply a small astronomical telescope of low magni- 

 fying power, e and 6 are Runsen burners, in which the sub- 

 stances under examination can be volatilised while their spectra 



FIG. 28. Spectroscope with reflected scale, a, prism ; b, collimator ; d, slit 

 plate ; e, e, Bun sen burners for volatilising salts ; /, observing telescope ; 

 g, photographic scale, illuminated by gas flame, h. 



are superposed by means of the comparison prism to be described 

 presently. 



Next a word as to the measurements necessary to enable us 

 to map or compare spectra with each other. There are several 

 ways in which this is commonly effected. In the instrument 

 represented in "Fig 28 there is a short tube carrying at its outer 

 extremity a small photographic scale g, which is illuminated by 

 a luminous gas flame h ; the light passing from the scale is 

 rendered parallel and thrown on the last surface of the prism 

 by means of a lens in the tube carrying the scale, and is 



