ABSORPTION- PRODUCED BY FLUORESCENCE. 



95 



the middle is seen through the eyepiece or on the focussing plate of the camera. The 

 two luminous portions are images of the parts of the surfaces a lf a',, cut off by the 

 horizontal slit immediately in front of them ; the screen rr and the paper screen 

 which separates A l5 A 2 , from A' 1} A' 2 , divide the image of this slit into two parts, and 

 produce the dark vertical line in the middle. 



By this arrangement, which was found to be on the whole the most convenient, 

 we have clearly the opportunity, not merely of comparing the two lights side by side, 

 which is absolutely necessary, but also of illuminating the surfaces A,, &c., 

 by the same source simultaneously and under similar conditions. 



ie. 8. 



(C) 



Any slight amount of light, chiefly blue, but partly red, which, having penetrated 

 the cobalt glass, may have been scattered by the fluorescent substance as false 

 dispersion (a term used by STOKES to designate the light scattered by irregular 

 reflection from small particles, which is always polarized in the plane of reflection, 

 and of the same wave-length as the incident light), can be eliminated, in consequence 

 of its being polarized, by interposing a NICOL'S prism or a pile of plates between the 

 horizontal and the vertical slits of the photometer. This method is open to the 

 objection that it would weaken the fluorescent light, and it has been thought 

 unnecessary to incur this disadvantage for two reasons. In the first place, the 



