Determination of Refractive Indices 3 



more frequent throughout the spectrum as shown by our equa- 

 tion above. In this way the complete record of dispersion of a 

 body may be obtained throughout the spectrum. Furthermore, 

 the bands superimposed upon the Fraunhofer lines as a scale 

 may be photographed and a permanent record made of the dis- 

 persion of the substance examined. 



To make this a practical working method was the object of 

 this investigation. 



A four-inch Rowland concave grating, 14,438 lines to the inch, 

 with Brashear mounting, and focal length of ten feet was used. 

 Fig. 1 shows the arrangement of the optical system. The slit 

 was illuminated by the direct rays of the sun, and an image of 



Fig. 1 



the same was formed on the wedge W by the lens L. In the 

 usual mounting of the Rowland concave grating, the slit or its 

 image should be placed over the intersection of the ways, in 

 order that the spectrum may be brought to a focus on the proper 

 focal curve. Instead of doing this, the light coming from the 

 slit S through the lens L was reflected by means of the prism p t 

 upon the wedge W , where the image of the slit was formed. 

 The two surfaces of W returned the light to the prism p 2 , which 

 reflected it upon the grating G. The distance of W from p i 

 and p 2 was such that the image in p 2 was at the intersection of 

 the ways, and thus the conditions for accurate focusing were 



131 



