776 Mr. F. C. Toy and Dr. J. C. Ghosh on the 



fitted with an iris diaphragm A, by means of which only 

 a small central circular portion of the lamp, about 1-2 mm. 

 in diameter, is used. A lens L is placed so as to project 

 a magnified image o£ this on to the slit SS of the spectro- 

 scope. The intensity of the image is very uniform, since it 

 is formed from such a small isolated patch of the source of 

 light. The Goldberg Wedge, W, is placed in front of the 

 slit and in contact with it. The length of slit used is ab ut 

 1*5 cm., and a special wedge was made up between two fused 

 silica optical flats by Messrs. Ilforcl Ltd. The intensity 

 incident on the wedge is varied by varying the aperture of 

 the projecting lens L by means of the adjustable stop B. 

 By keeping the aperture small and the focal length large, 

 so that the curvature of the lens surfaces is small, the 

 intensity of the image is almost exactly proportional to the 

 square of the aperture of the lens. The current through 

 the lamp is of course kept constant during each experiment. 

 A metal filament lamp is used for experiments at the red end 

 of the spectrum, since there are no lines in the mercury arc 

 suitable for measurement in this region. The intensity is 

 varied in this case by changing the distance of the light 

 source from the wedge. 



This apparatus is used by taking two spectrographs through 

 the wedge on the same plate with two differing intensities 

 in known ratio and with the same time of exp sure. From 

 this negative the gradation at several wave-lengths can be 

 determined as follows. Let A A be the gradation of the 

 wedge for any wave-length X, and the two incident inten- 

 sities be i! and I 2 . Let the densities at the two points at 

 which there is the same intensity (L) transmitted in each 

 case be D : and D 2 , and the distances of these points from 

 any arbitrary fixed point on the wedge be x { and x 2 . At the 

 images of these points at any particular line of the spectrum 

 the same effect will be produced, so that on development the 

 densities will be equal. This equality is independent of 

 the exposure, the kind of plate used, or the treatment it 

 receives. 



From the definition of density, 



L=I lX 10-^ = 13x10-^, 



.'. log 10 Ii/I 2 = D 1 ™D 2 = A A xOi' 1 -^ 2 ). 



Therefore 



Oi~tf 2 ) = £- Xtogioli/Iz (1) 



