272 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



The angles of the prisms being very small, we can replace the 

 sides of the angles by their circular measures, thus : — 



^ = 1+A (3D. 



By this formula the indices given in Table VII were calculated. 



Plate 16 shows graphically the contents of Table VII. The 

 abscissae are wave lengths in /x /x units, while the ordinates are 

 indices of refraction. 



For three reasons the results are not so good in the absorption 

 band as in the red and blue parts of the spectrum. These are : 



1st. To transmit the yellow light the prisms must be very 

 thin, and hence, of small angle. This increases the percentage 

 error in measuring the angle of the prism as well as the devia- 

 tion. 



2d. The image of the slit made by the deviated ray was not 

 as sharply denned in the absorption band as in the transparent 

 parts, a diffraction haze appearing around the image due to the 

 narrow portion of the prism through which the light could pass. 



3d. Any slight error in measuring the wave lengths would 

 affect the result to a larger degree in this portion of the curve 

 than in the other parts. 



