440 Prof. Ch. V. Zenger on a new Spectroscope. 



Any contrivance by which this loss could be avoided and yet 

 the same amount of dispersion be secured would be very advan- 

 tageous for spectrum-observations on feeble, and even on strong 

 light. 



For this purpose I employ only a single prism of the least- 

 absorbent material and the greatest dispersion attainable. It 

 seemed to me that a prism of pure transparent ice-spar would 

 best fulfil this condition, as its index of refraction is : — 



n b — 1*6531, 

 % = 1-6833 , 



A/i= 0-0302; 



so that its dispersion is greater than that of crown glass, and but 

 little less than that of flint glass, while it absorbs very little of 

 the extreme red and violet rays. 



To explain the action of the new spectroscope, let ABC 

 (fig. 1) be the section of a rectangular prism of spar; the solar 



Fig. 1. 



ray Sw2 falling at right angles on the surface AB, the angle of in- 

 cidence G> is equal to the angle of the prism. After refraction 

 the red ray falls in the direction m Q m on the surface of a cylin- 

 drical mirror and is reflected from it in the direction m B ; and 

 the violet ray will be reflected, at a greater angle, in the direc- 

 tion m 1 V. 



The diagram shows at once that the effect of the cylindrical 

 surface amm x is to produce an increase of the angle of disper- 

 sion m m Q m, = <£ to R h V=i|r. It is easy to account for the in- 

 crease, if the angle of the refracting prism, the distance between 

 the prism and the mirror, and the radius of curvature of the 



