366 



S. P. Langley — Energy and Vision. 



Each, reading of the logarithms in the slider is taken when 

 certain figures become discernible in the light in question, and 

 is the mean of three independent observations, taken consecu- 

 tively. In order to find the wave-length by means of the 

 prism we must prepare a table or a graphic construction, de- 

 duced from an examination of the special prism employed, 

 showing the wave-length corresponding to the position of mini- 



B=<ffX 



mum deviation of each ray. Figure 3 is such a graphic con- 

 struction derived from our own observations of the constants 

 of the prism employed, and Table II gives the approximate 

 values of the tangents to the curve by means of which we pass 

 from the prismatic to the normal scale. 



