78 Dr. J. W. Draper on a new 



This, though permitting of pleasant working, had not the ex- 

 actness of the method of distances. 



Such are the results obtained from the prismatic dispersion 

 of gaslight. I completed this part of the investigation by an 

 examination of sunlight. For this purpose I resorted to the 

 foregoing principle, introducing a beam of sunlight reflected 

 from a heliostat through a slit. The spectrum of this was 

 thrown upon a paper screen, so placed that by opening or 

 closing an adjacent window-shutter the light of the sky in 

 greater or less quantity could fall upon the paper, and act as 

 an extinguisher. When the shutter was fully opened, the 

 spectrum was quite obliterated; and on gradually closing it so 

 as to diminish the extinguishing light, the red region first 

 came into view, the other colours following in the order of 

 their refrangibility, the extreme violet appearing last. On 

 reversing the movements of the shutter the colours disappeared 

 in the reverse order, the red disappearing last. 



At the moment when the red was approaching extinction 

 there always existed on its more refrangible side a gleam of 

 greyish-green light. In was in the position of that greenish 

 gleam which appeared, as I have described, when gaslight 

 was examined. Its colour recalled to my mind the faint 

 greenish-grey light I had seen when a strip of platinum was 

 ignited by a feeble electric current, as described in my memoir 

 of 1847, above referred to. 



Subsequently I constructed a camera having two apertures 

 in its front. Through one of them (by a suitable arrangement 

 of a heliostat, slit, direct-vision prism, and convex lens) a solar 

 spectrum was formed on the ground glass. Through the 

 second aperture, which was about an inch square, covered with 

 a glass ground on both faces, an extinguishing beam of sun- 

 light passed. This ground glass served to disseminate the 

 extinguishing light uniformly over the spectrum. I could 

 regulate its power by varying the size of the aperture through 

 which it came, by means of a slide. 



It is needless to give details of the results obtained by this 

 instrument. They were identical with those described in the 

 foregoing paragraphs. 



It might be supposed that the irrationality of dispersion of 

 different prisms would influence the results perceptibly. 

 Accordingly I tried prisms of different kinds of glass and 

 other transparent substances, but could not find that this was 

 the case. In all the extinction began in the violet and ended 

 in the red. 



Nor did there seem to be any difference when the effect 

 was viewed by different eyes. To all, irrespective of age or 



