Direct Vision Spectroscopes by Double Internal Reflection. 447 



-which prisms for direct vision spectroscopes can have, and 

 beyond which the second reflection disappears. 



"With a glass of high refractive power, as, for example, 



1*66, the right-angled 

 prism, and the compound 

 spectroscope composed 

 of two such prisms (Figs. 

 8, 9) holds a mean place 

 between the extreme 

 forms of the two pre- 

 ceding tables — the an- 

 gle A in this prism being 

 equal to 20° (between 

 15° and 30°), and the 

 angle c is 90° (instead 

 of 75° or 120°). 



This form of prism 

 is more simple of con- 

 struction, and may be 

 distinguished from the 

 other more complicated 

 forms by the name of the " three-to-one right-angled prism," 

 because for common glass, the longest side is to the shortest 

 side of the triangular section of the prism, in the ratio of very 

 nearly three to one. The following are the values of the angle 

 A in this prism, for every different kind of glass : — 



Table III. 



Index of 



refraction 



of the glass. 



Angle A, 



for a 3:1 



prism. 



Angle c, of 

 ditto. 



Forms 



of 3:1 prism for different 

 kinds of glass. 









( c n- 





1-4 



15° 0' 

 16 52 



90° 0' 

 90 



/^ 



j^T -*^ 



1-5 



I B 



A 



Fig. 10. 



1-6 



18 47 



90 



C 





1-7 



20 25 



90 



( /£ 



*>«^^ 



1-8 



21 52 



90 



/ 



SPtx^ 





u 



Fm. 11. 



Scale of angles for the three-to-oae right-angled (direct-vision) prism for 

 different kinds of glass, j 



