270 Direct 'Vision Spectroscope of one kind of Glass. 



The second and third prisms in the plan have been drawn 

 of the same shape as the first and fourth, but as one of their 

 three sides is not, in this instrument, employed, thev may be 

 much curtailed. I have thought it proi>er to draw them as 

 shown that the parallelism of the various sides, where it 

 exists, may be strongly brought out. 



In conclusion it may be stated tbat in the actual instru- 

 ment shown the glass has been selected so as to give 

 admirable visibility from A to H, the line F very nearlv 

 corresponds with the selected ray, and there is a dispersion of 

 18~ 20' betw^een A and G. 



