Page:Proceedings of the Royal Society of London Vol 60.djvu/22

14 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 he more convenient than that adopted, hut the time I had at my disposal prevented my carrying 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

Photographing with Monochromatic Rays.

of light passed through the slit S of the collimator on to the centre of the collimating lens L2. A convex lens Lx of shorter focus than L2 was placed in the path of the rays, and so adjusted that a real image of the poles was formed on L2. These passed through the lens L2 as nearly parallel rays and struck upon the prism, and then passed through the remainder of the apparatus as sketched in fig. 2, whei’e M is the prism, L3 a lens to bring the rays to a focus as a spectrum on after passing through a camera, A. LA is a lens, shown in the figure connected with a camera, B, which brings the image of the prism and the bright image cast on it to a focus at P. By placing a slit S2 in the spectrum, the image cast on P will be as monochromatic as the light coming through the slit. Lx should be of such a focal length that it should be as near the slit as possible. W ith this arrangement it is very cnrious 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