450 



Messrs. G. D. Liveing and J. Dewar. [Dec. 9, 



spectroscopes with compound prisms have the disadvantage that the 

 dispersion of the red end of the spectrum is small ; less in proportion 

 to that of the blue end than in spectroscopes with simple prisms. 

 Also no measurements of lines can be made with them, except by 

 means of a scale fixed in the field of view, which it is often difficult to 

 see for want of illumination. 



Some time since (* Roy. Soc. Proc.,' vol. 28, p. 482) we brought 

 under the notice of the Society a direct vision spectroscope on 

 Thollon's plan, which had not the faults of the instruments with 

 compound prisms. It gave a dispersion equal to that of two prisms 

 of 60°, and excellent definition, but the number of reflecting and 

 refracting surfaces which had to be truly wrought was rather large, 

 and the movement of the prisms by a screw made measurements with 

 it slow. 



Since then we have tried a spectroscope with one of the Astro- 

 nomer Royal's half prisms, but we found it impossible to get good 

 definition with the half prism for more than a small part of the 

 visible spectrum ; and in consequence faint bands near either end of 

 the spectrum were quite invisible with this instrument. 



The arrangement we have now to describe was intended to obviate 

 the defects of the others. 



It has three prisms symmetrically arranged, the middle one 

 serving both for refraction and reflexion. The course of a ray 

 through the prisms is indicated in the annexed diagram. A ray Im 

 in the line of the axis of the collimator meets the first prism ABC in 

 m, is refracted at m aud n, meets the second prism EGrB at o, and is 

 then refracted, undergoes two internal reflexions at p and q, and is 

 refracted out at r ; it is then refracted through the third prism DEF 

 at s and t, and emerges in the direction tu y which is a prolongation of 

 its original direction Im and coincides with the axis of the observing 

 telescope. The prism EGB is fixed, the other two prisms are movable 

 about axes parallel to their edges passing through the points m and 

 t. They are rotated simultaneously in opposite directions by a pair. 



