1894] 



of prisons for a stellar spectroscope. 



139 



These two prisms are relatively fixed as shown in the ac- 

 companying section with the faces AB and DE parallel (or 



Prisms relatively fixed. Combination capable of being turned round G. 



approximately so), the edges of the prisms are also parallel and 

 further the edges G and F lie in a plane GGF which passes 

 through the middle, G, of the face AB and is perpendicular 

 to AB. 



The combination thus formed is symmetrical about the plane 

 GGF and is made capable of being turned about an axis which is 

 the intersection of the plane GGF with the face AB. Thus in the 

 figure, the section turns in the plane of the paper round G. 



Light from a collimator falls on the face AB as indicated in 

 the figure (the central ray however falls on G), and the incidence 

 is suitably adjusted by turning the combination about G; the 

 usual telescope is adjusted to view the spectrum. The light 

 from the collimator suffers deviation and dispersion in passing 

 through the prism of refracting angle A; it is then twice reflected 

 within the reflecting prism and again suffers deviation and dis- 

 persion in passing through the prism of refracting angle B. 



The spectrum seen with the telescope is therefore similar to 

 what would be seen in a two-prism spectroscope whose prisms 

 were of the same material as that used for the prism ABG and 

 had angles equal to A and B respectivel3^ 



The telescope receives, when it and the prism-combination are 

 properly adjusted, not only the light which has passed tlirough the 



