LIGHT. 119 



a spectrum fall upon the screen at S. This spectrum 

 will be seen to be traversed by a large number of black 

 bands distributed throughout the whole length of it. 

 If the plate of mica be very thin and white there may 

 be as few as eight of these striae, but if it be thicker 

 their number will be largely increased. 



The room will need to be made as dark as possible 

 for this experiment, as the spectrum will not be very 

 bright at best, and it therefore cannot be enlarged. 

 If the length of the spectrum exceeds a foot it will be 

 quite dim. These lines, however, can be seen to great 

 advantage by placing the eye close to the prism when 

 in its place as shown above. 



If the spectrum of the light reflected from mica be 

 received upon a paper screen painted over with a solu- 

 tion of quinine and thus rendered fluorescent, such 

 interference striae will make their appearance in the 

 ultra violet part of the spectrum. 



SPECTRUM ANALYSIS. 



To project the spectrum of any substance what- 

 ever it must be heated until its vapor is brilliantly 

 incandescent. The heat of the electric arc is best 

 for this work as every substance is vaporized there. 

 The lime light may be used to exhibit the prin- 

 ciples of spectrum analysis, but its heat is insufii- 

 cient for most of the metals. The characteristic 

 lines of Sodium, Calcium, Lithium, Barium, Stron- 

 tium, Potassium, and Copper may be tolerably well 

 exhibited with a lantern furnished with oxyhydrogen 

 jet and gases. 



I St. To exhibit the spectrum : — 



Produce the lime light as you would for common 

 projection. Remove the objective and place at the 



