1902.] Refractive Indices of Fluorite, Quartz, and Calcite. 329 



30*9, 61*8 : these values are expressed in the curve of fig. 5, which is 

 therefore a curve expressing roughly the luminosity of different parts 

 of the same spectrum according to the evidence afforded by nicker. 

 For the sake of comparison the luminosity of the different colours as 

 measured by Vierordt is indicated by a dotted curve, and it will be 

 seen that the two curves practically coincide, except for the extreme 

 parts of the spectrum ; it should, however, be stated that Vierorclt's 

 curve has been drawn by making his value for the luminosity of the 

 yellow coincide with that given for this colour by the continuous 

 curve in the figure, and reducing his other values in the same propor- 

 tion : the reduced values are then placed within the limits of the colours 

 to which they are'assigned by Vierordt. Moreover, Vierordt's curve is 

 for a solar prismatic spectrum, whilst the other is for incandescent 

 lime, used with a grating.* Lastly it should be remembered that the 

 disc was very feebly illuminated in the violet and extreme crimson, 

 and also, for the small values of the white coloured sector, in the blue, 

 blue-green, and vermilion, so that the alteration of the relation 

 between n and log I (indicated in fig. 2 by the steeper line for feeble 

 illuminations) should probably be considered: if this is so, the two 

 curves will be brought into still closer union. The writer postpones 

 the consideration of this matter, until he has had the opportunity of 

 trying further experiments with such feeble illuminations. 



" The Eefractive Indices of Fluorite, Quartz, and Calcite." By 

 J. William Gifford. Communicated by Silvanus P. Thomp- 

 son, F.E.S. Keceived February 5, — Eead February 13,— 

 Eeceived in revised form May 7, 1902. 



1. Method of Observation. — Measurements of fluorite, quartz, and cal- 

 cite have been made by Eudberg, Mascart, Cornu, Sarasin, Glazebrook, 

 Van der Willigen, Vogel, Pulfrich, Eubens, Bailie, and many others. 

 Those now offered were originally undertaken with the view of further 

 extending the range and accuracy of lenses constructed of these sub- 

 stances. A new method of obtaining the refractive indices has been 

 adopted. Each of the angles of the prisms used was as nearly as 

 possible 60°. When this is the case it is sufficient to measure the devia- 

 tion of light of a definite wave-length at each angle in turn ; the 

 mean of these deviations may be taken as the deviation corresponding 



* C. Vierordt, < Pogg. Ann.,' vol. 137, p. 200. 



