110 



S. B. Williams — An Achromatoscope. 



each other, the image of the illuminated slit is thrown on the 

 two wedge-shaped films, f x and f„. The two wedge-shaped 

 films mav be moved parallel to their lengths by means of 

 screws which extend forward to the observer seated at the 



Fig. 7. 



Fig. 8. 



-k/- 



eye-piece of the telescope. This affords a means of changing 

 the order of interference in either one of the films and of 

 bringing the spectral bands into coincidence as nearly as pos- 

 sible. The two sets of light rays from the two surfaces of the 

 films are now returned to the right angled prisms, jp s and p t . 

 These two prisms are arranged as p 1 and^> 2 , and throw the light 

 through the lens, L„, so that the image of the image of the 



