318 



Sir W. de W. Abney. On the Variation in 



showing that the glass of the prisms absorbed a certain proportion 

 of the different rays as they passed through. It appeared probable 

 that if the length of the J-inch-wide slit were placed vertically in 

 the patch of light {i.e., parallel to the edges of the prism) no difference 

 in absorption would be found. Such proved to be the case ; the 

 exposure through the slit and the patch of light without the inter- 

 vening sectors gave a uniformly dense deposit, and when the sectors 

 were replaced the densities given by the three 40° exposures were the 

 same. On each plate exposures were given to four different colours, 

 the total exposure varying in each case according to the colour ; a 

 single exposure was also given to some colour without the sector, 

 and an exposure to an amyl-acetate lamp was also given. The 

 following tables give the results obtained, and fig. 8 the results 

 shown diagrammatically of Table XV, and the combined results are 

 shown in fig. 5. 



Table XV.— Densities. 



Aperture 

 of 

 sectors. 



Scale number. 



55 *4 

 (A 6277) 



40-6 

 (A 5300) 



31 -4 



A 4901 



22-2 

 A 4584 







5 



35 



37 



45 



45 



10 



42 



44 



60 



65 



20 



57 



60 



85 



90 



40 



82 



80 



122 



125 



80 



130 



119 



160 



160 



160 



178 



159 



197 



195 



Table XVI. — Densities. 







Scale number. 





Aperture 











of 











sectors. 



39-3 



25 



15 



6-6 





(A 5320) 



(A 4675) 



(A 4377) 



(A 4162) 







5 



53 



75 



75 



53 



10 



67 



98 



100 



60 



20 



89 



121 



125 



82 



40 



115 



144 



150 



107 



80 



140 



167 



174 



133 



160 



166 



190 



198 



157 



