S. R. Williams — An Achromatoscqpe. Ill 



slit, S„ is formed at S 2 where the slit of the spectrometer ordi- 

 narily comes. The upper half of the slit image is light from 

 one film and the lower half from the other wedge-shaped film. 

 The lenses, L, and L„ and the prisms, p,, j? 2 , p t , and _£> 4 should 

 be good ones. 



In fig. S is shown a very simple device for mounting two 

 films in comparison work. The films are fastened edge to edge 

 in a frame, F, which is placed before the slit of the spectro- 

 meter at an angle of about 45 degrees with the axis of the col- 

 limator tube. The sunlight reflected from the upper film may 

 be made to illuminate the upper half of the slit while that 

 from the lower film will illuminate the lower half of the slit. 

 The channeled spectra in lig. 2 were taken in this way, which 

 of course does not permit shifting of one film with respect to 

 the other although such an arrangement could be easily made. 



Optical concerns investigating the various kinds of glass 

 which they use in achromatic combinations ought to find an 

 achromatoscope, such as- described in the foregoing pages, a 

 convenient method for rapidly testing the various specimens of 

 glass submitted. The figuring of thin wedges ought to be no 

 more laborious than an ordinary prism and once secured does 

 not require such elaborate measurements as the prism method. 



Summary. 



1. The theory for determining the optical thickness of a 

 film without knowing either its actual thickness or its index of 

 refraction has been developed in a simple manner. 



2. A method for the study of the dispersion of a substance 

 has been developed, using a wedge-shaped film when the 

 refracting edge is normal to the slit of the spectrometer. This 

 method is applicable to anomalous dispersive substances. 



3. Application of channeled spectra to a study of achromatic 

 combinations has been pointed out and a practical device 

 developed for rapidly testing various kinds of glass for achro- 

 matic combinations. 



Physical Laboratory, Oberlin College, Oberlin, Ohio. 



