Differential Double Refraction. 543 



a scale, M, graduated in fifth millimetres, was projected into 

 the telescope, H I, and the position of the bands plotted. 

 The setting of the grating in order to reflect the scale into 

 the eyepiece prevented the formation of a strictly normal 



Fig-. 2. 



spectrum, but the spectrum observed was sufficiently so to 

 indicate the true character of the distribution of the bands. 

 Fig. 3 shows different relative positions of the bands in selenite 

 and quartz in the spectrum. The first system shows that the 

 orders 65 selenite and 66 quartz coincide better in the orange 

 and red than the third system of 67 selenite and 66 quartz ; 

 but the latter coincide much better in the green and blue. 

 The last named orders give a good coincidence between yellow 

 and blue where the light is intense. The systems show that 

 in the normal spectrum the bands are furthest apart in the red 

 and narrowest in the blue. This agrees with the theory of the 

 distribution of these bands as given by Prof. Brace. The 

 curves in fig. 4 indicate the relation of the order to the devia- 

 tion in the irrational refraction-spectrum, the image of the scale 

 representing abscissae and the number of the black interference- 

 bands the ordinates. The band coinciding with the Na line is 

 represented as the zero band. The orders used were 75 for 

 mica, 88' 5 for Iceland spar, 187 for selenite, and 190 for quartz. 



