Phase Change by Reflection 



5 



was cast on the slit of a four-foot Rowland grating spectroscope. 

 This method requires a source giving a continuous spectrum. 

 Sources fulfilling this requirement and also giving sufiiciently in- 

 tense radiation in the ultra-violet were not available. With a 

 Nernst glower and with exposures eight hours in duration it was 

 found that the limit into the ulti-a-violet to which observations 

 could be extended was 340 /x/x. 



Second Method 



For most of the measurements the following method was used 

 which, since it made possible the application of a discontinuous 

 spectrum, permitted measurements to be extended farther into the 

 ultra-violet region. 



Thin glass wedges were selected from several boxes of cover 

 slips and from a small supply of thin glass plates that were at 



(^cailn^ 



.am«r*a 



Fig. 2. 



hand, all being discarded that failed to show straight bands of 

 sufficient breadth when illuminated by monochromatic light. The 

 breadth of band that gave the best photograph in the shorter wave- 

 lengths was found from a few trials. These were then coated 

 on the back with the substances whose phase change was to be 

 investigated. Then a strip of the coating was removed in a direc- 



171 



