ABSORPTION PRODUCED BY FLUORESCENCE. 93 



difference due to scattering did not appear to be more than 1 part in 8 at the 

 maximum, and was for the purpose of these experiments altogether inappreciable. 



DJESCBIPTION OF APPARATUS. 



1. Fluorescent Cnlcs. 



2. Photometer. 



1. The blocks employed are each 1 centim. cube, and are kept together by two 

 indiarubber bands. The two front cubes, A^ A.\ (fig. 2), are separated from the two 

 back ones, A,, A' 2 , as has already been pointed out on p. 89, by a plate of glass of 

 thickness (about 2 millims.) sufficient to stop the fluorescence-exciting rays scattered 

 by one pair of cubes from entering the other, whilst the cubes Aj and A 3 are divided 

 by a black paper screen from A.\ and A' s , so that no light could pass sideways from 

 Aj or A 2 to A.\ or A' a . 



The horizontal surfaces A x , A 2 , A' ]( A' 2 , are those which are illuminated, whilst the 

 light studied is that emitted by the surfaces a l} a\. 



The surfaces 1> V , d L ; V ^ d\ ; 6 2 , c. 2 , d 2 ; l>'.,, c' 2 , d' 2> as well as the lower horizontal 

 surfaces parallel to A 1; A.\; A 2 , A' 2 , are roughened, so that there is no regular reflec- 

 tion from them, and thus are more or less opaque. But the surfaces ct it c ] ; a\, c\ ; 

 a. 2 , a' 2 , are perfectly transparent, as also the upper horizontal surfaces A. lt A\ ; A.,, A';. 



When Aj, A' 1; A 2 , A' 2 , are all uniformly illuminated by violet or ultra-violet rays, 

 the fluorescence of each block ought to be the same, if we disregard the very refined 

 correction that the blocks A 1} A.\, have each two transparent vertical surfaces, namely 

 !, Cp and a\, c\, whereas the blocks A 2 and A' 2 have each only one, a. 2 and '.,. At 

 any rate, since the rays enter vertically, and therefore parallel to these surfaces, any 

 difference that may possibly arise in this way in the intensity of the scattered rays 

 would for all practical purposes be quite immaterial. 



We have seen (p. 92) that the error, when sought for. was found to be quite inap- 

 preciable and lies within the much larger errors of observation. Apart altogether 

 from this consideration, the fluorescent light emitted horizontally from the surfaces 

 rtp ct'j, ought to appear the same, if all is right, and this is ascertained by the photo- 

 meter before an experiment is made. 



2. Fig. 7 is a plan of the apparatus showing the position of the fluorescent 

 cubes, &c., in the photographic experiments. The cubes are placed Aj, &c., and 

 are illuminated by light incident vertically upon them either from the spark in 

 air between cadmium electrodes or the electric arc. The source of illumination was 

 almost invariably the former in the photographic determination, the poles being 

 about 15 millims. apart, and 2 centims. above the fluorescent surface. The spark 

 was obtained by the discharge of a Leyden jar charged by the terminals of the 

 secondary of a 10-inch Apps coil, 



