Anomalous Dispersion of Carbon. 409 



touching it), very uniform prismatic deposits of smoke were 

 secured. Viewed in reflected light the films showed Newton's 

 interference colours, and the prisms were judged by observing 

 whether the coloured bands were straight and properly spaced. 



A suitable prism having been selected, the plate was 

 covered with a piece of black paper perforated with two 

 rectangular openings of equal width. 



One of these openings, which was several times higher 

 than the other, was over the prism, while the other covered 

 clear glass close to the refracting edge of the prism. The 

 object of using diaphragms of the same width but of different 

 heights was to secure similarity of the deviated and un- 

 deviated images of the slit. The width of the central 

 maximum depending on the width of the opening alone, the 

 height could be increased to allow the passage of more light 

 in the case of the dark prism. 



The plate was then placed in a large direct-vision spectro- 

 scope (from which the prisms had been removed) and the slit 

 illuminated alternately with red and blue light. A very 

 intense light being necessary the electric arc was used, the 

 slit being covered alternately with deep ruby glass and a 

 glass trough containing a strong solution of cuprammonium 

 combined with a cyanine film. Prismatic analysis was found 

 to weaken the light too much. 



The diaphragms were covered alternately, and readings 

 taken of the position of the slit image with a filar micrometer. 

 The following readings were obtained : — 



Red. Blue, 



Distance between dev. 



Distance between dev. 



and undev. image in 



and undev. image in 



divisions of micrometer. 



divisions of micrometer. 



55 





40 



60 





50 



59 





41 



58 





48 



57 





45 



58 





46 



53 





43 



56 





39 



55 





44 



61 



mean. 



40 



57-2 



43*6 mean. 



In order to see how these readings agreed with the devia- 

 tion calculated from an interferometer determination of the 

 Phil. Mag. S. 6. Vol. 1. No. 4. April 1901. 2 E 



