MAGNUSSON — ANOMALOUS DISPERSION OF CYANIN. 



275 



per, furnished with two small apertures, was mounted over the 

 plate, one aperture exposing the prism, the other, clear glass. 

 Thus, the light arrived at the sensitive plate by two paths, one 

 direct and the other deviated by passing through the cyanin 

 prism. Therefore, two sets of the Fraunhofer lines were found 



------31' 



Fig. 5. 



on the plate, of which two sample photographs are seen in plate 

 19, figures 1 and 2. The upper set of lines was formed by 

 rays coming through the prism and the lower set by the direct 

 path. For making these photographs the angle of the prism was 

 2' 51.5", the distance from the prism to the plate was 177.6 c. m., 

 while the time of exposure for the direct ray was three minutes 

 and for the other seven minutes. 



The distance between a given line in one set and the corres- 

 ponding line in the other set was measured by a filar micrometer 

 attached to the eye-piece of a microscope. Let X be this dis- 

 tance in centimeters ; let d be the distance from the plate to the 

 prism in centimeters, and let a be the angle of the prism 

 in seconds of arc, then the index of refraction for the wave 

 length used is given by the following formula : — 



, , X 360 

 w=1 + 2lFd^ 



(32). 



