20 



A STUDY OF THE ABSORPTION SPECTRA. 



and potassium cyanide. The solution should be gently heated. The lower 

 layer of liquid is used for the cement and is heated for several hours at a 

 temperature of 160 under the pressure of 100 pounds per square inch. 



D is a brass box holding the trough T. D is filled with oil and is placed 

 in a water-bath whose temperature can be varied between and 90 C. 

 The path of the beam of light is then from the Nernst glower N or spark 

 to the quartz prism P'. The light is totally reflected from the hypothenuse 

 face of this prism through the solution to P. This prism also has its hy- 

 pothenuse face backed by an air-film, so that the light is totally reflected 

 upwards to the concave speculum mirror at M. M focuses the light on the 



slit of the Rowland concave grating spectroscope, G being the grating and 

 C the focal curve of the spectrum. The prism arrangement was designed 

 by Dr. John A. Anderson. 



When the quartz prisms were being set up for temperature work a very 

 peculiar set of images were obtained that the writers have not yet fully 

 understood, although the optics is probably quite simple. When a Nernst 

 filament is observed through one of the prisms so that the incident beam 

 of light is symmetrical to the prism, there is but one image of the filament 

 to be seen. If now the prism is rotated in general three images will be 

 seen. For a certain position of the prism the three images are of practi- 

 cally the same intensity. 



