14 Photographing with Monochromatic Rays. 



plate moving across the slit in the spectrum at the requisite rate for 

 the various impressions made by the different parts of the sun's image 

 to coalesce. It had struck me some time since that the method thus 

 indicated nearly eleven years ago might be more convenient than 

 that adopted, but the time I had at my disposal prevented my carry- 

 ing out a continuation of my experiments. Recently I have had 

 occasion to take up this subject for a rather different purpose, and as 

 the method seems to have been untried, I give it in more detail than 

 I did then. 



My investigation called for a determination of the proportions of 

 various rays emitted by the various parts of the carbon of the positive 

 and negative poles of an electric arc light, and for this purpose the 

 system of forming monochromatic images was found to be useful. 

 The points of the electric light EL (fig. 1) were placed so that a beam 



of light passed through the slit S of the collimator on to the centre of 

 the collimating lens L 2 . A convex lens L! of shorter focus than L 2 was 

 placed in the path of the rays, and so adjusted that a real image of the 

 poles was formed on L 2 . These passed through the lens L a as nearly 

 parallel rays and struck upon the prism, and then passed through the 

 remainder of the apparatus as sketched in fig. 2, where M is the 

 prism, L 3 a lens to bring the rays to a focus as a spectrum on ub 

 after passing through a camera, A. L4 is a lens, shown in the figure 

 connected with a camera, B, which brings the image of the prism arid 

 the bright image cast on it to a focus at P. By placing a slit S 2 in 

 the spectrum, the image cast on P will be as monochromatic as the 

 light coming through the slit. L : should be of such a focal length 

 that it should be as near the slit as possible. With this" arrangement 

 it is very curious to watch the variations in the brightness of the arc 

 and of the flame which accompanies the movement of the slit through 

 the spectrum, and as each variation can be photographed on a Cadett 

 polychromatic photographic plate, we can obtain records of all that is 



