94 ME. JOHN BDRKE ON THE CHANGE OF 



The photometer, fluorescent substance, and source of illumination in the photo- 

 graphic experiments were enclosed in a wooden box, shown in the figure. The 

 photometer was at one end, and the fluorescent substance and illuminating arrange- 

 ment at the other. 



Fig. 7. 



KK represent the electrodes, A I} &c., the fluorescent cubes, and P the photometer, 

 consisting of two vertical slits, s, s', and a double convex lens, L ; while tt is a card- 

 board vertical screen, with a horizontal slit, 3 millims. wide, placed immediately in 

 front of the vertical surfaces of the cubes (the slit cannot be shown in the figure). 

 This screen admits of being moved up and down, so that the light at various depths 

 below the illuminated surface may be studied. The usual depth of the upper edge 

 of the slit below the horizontal surface of the fluorescent cubes was about 3 millims. 

 Another vertical screen (rr), G centims. in length, extends along the whole length of 

 the box from the photometer to the other screen, and divides the horizontal slit into 

 two parts, the light from each part being allowed to pass only through the corre- 

 sponding vertical slit of the photometer. 



The vertical slits s and a' were adjustable with micrometer screws. 



The lens L (of focal length 4'5 centims.) served to form an image of the horizontal 

 slit in the focal plane of the eyepiece, or on the focussing glass of the camera, which 

 admitted of being raised or lowered, so that, when necessary, a number of photographs 

 could be obtained successively on one plate. The photometer contrivance communi- 

 cated with the camera by means of the conical tube T, leading up to a front slide which 

 remained fixed whilst the camera itself moved up and down (fig. 8). The distance 

 between the lens L and the focal plane of the eyepiece was 18 centims. The whole 

 of the interior of the apparatus was blackened, so that not the slightest trace of 

 reflection could be perceived. 



A screen of blue cobalt glass was interposed between the source of illumination and 

 the fluorescent substance so as to cut off the less refrangible rays. When the appa- 

 ratus is in use a horizontal green band with a dark vertical line or narrow band in 



