Ill 



410 On the Anomalous Dispersion of Carbon. 



retardation at the base of the prism, the plate was set up in 

 front of one of the mirrors of the instrument, a plate of equal 

 thickness being introduced in the other optical path as a 

 compensator. The fringes sloped abruptly down across the 

 face of the prism, the displacement at the base being 1*7 of 

 a fringe for the double transit of Na light, or '85 for a 

 single passage. The retardation is therefore *00059 x '85 or 

 *0005015 mm. The width of the prism was 2*24 mm., 

 therefore the angle through which the wave-front should be 

 turned is given by 



•0005105 



tan$ = 



2-24 

 0=46". 



The actual deviation as observed in the spectroscope was 

 determined as follows : — 



Calibration of the filar micrometer showed that one com- 

 plete revolution of the head (100 divisions) corresponded to 

 *248 mm.; 50 divisions, the mean between red and blue 

 corresponding approximately to the deviation for sodium 

 light, represents a shift of '124 mm. 



The focal length of the telescope was 600 mm., and the 

 deviation is given by 



, a * 124 

 W = -600 



0=43", 



which agrees vel'y well with the calculated deviation of 46". 



While the results obtained are what we should expect if 

 the diffraction theory were true, they do not of course 

 prove it ; and it will not be safe to draw deductions until 

 more accurate results have been obtained with the metallic 

 deposits obtained from the incandescent carbon filament. A 

 method of compensation has been devised, which will make 

 observations with much thicker films possible. 



In conclusion I must thank Dr. C. E. Magnusson for much 

 valuable assistance in photographing and measuring the 

 fringes. 



Physical Laboratory of the University of 

 Wisconsin, Madison. 



