482 Mr. F. Twyman on Improvements in the 



for Dr. C. E. Kenneth Mees in 1904, and embodies many- 

 suggestions of his and of Mr. S. E. Sheppard's. The Optical 

 Train is the same as that of the Hiifner Spectrophotometer 



Fig. 3. 



as made by Albrecht ; with the exception that instead of the 

 dispersion prism being the usual prism of 60°, the now fairly 

 well-known constant-deviation prism is employed. 



Fig. 4. 



The prism is shown in fig. 4. It may be considered a& 

 built up of two 30° prisms and one right-angled prism from 

 the hypotenuse of which light is internally reflected as shown 

 in the figure. The prism is, however, made in one piece. This 

 construction of prism has the very important mechanical 

 advantage that the telescope and collimator are both fixed, 

 being rigidly attached to a strong cast-iron base. The passing 



