PRISMATIC SPECTRUM ON BROMIDE OF SILVER. 117 



glass equiangular prism, d, aud was dispersed by it, the spectrum, e f, being received on 

 the table; this spectrum was about three inches long. And now a piece of paper, im- 

 bued with bromide of silver, was placed to receive it, with the intention of ascertaining 

 how far the discoloration would extend. In the course of five minutes a very marked 

 change had taken place, and, on examination, it was found that the deepest tint had 

 been occasioned where the violet blended with the indigo rays ; beyond this, even in 

 the dark space beyond the spectrum, there was a stain, as also as far in the spectrum 

 as where the greeu light merged into the yellow, an effect represented in Jig. 66, a a, 

 b b, being the spectrum ; during this experiment the spectrum was kept stationary. 

 Again, a column of light, three inches in diameter, converging from a convex lens (a a, 

 Jig. 67), was intercepted by a screen of pasteboard, b b, which had a circular aperture 

 in it half an inch in diameter; this screen was placed at such a distance from the 

 focus, that the circular section of the cone of light was half an inch in diameter, and, 

 therefore, passed exactly through the aperture ; a piece of the prepared bromide pa- 

 per was then fastened on the back of the screen, so as to receive the condensed 

 ravs which passed the aperture. In a few moments a black spot appeared about the 

 central parts of the paper, and at the end of the experiment there was an intensely 

 black circle, surrounded by a brown, ring-like penumbra, as in Jig. 68; the diameter of 

 the black spot being three quarters less than that of the aperture through which the 

 light passed. 



446. INTERFERENCE OF THE CHEMICAL RAVS. Under certain circumstances, two 

 aerial vibrations, each of which, if separately striking the organs of hearing, would pro- 

 duce a musical sound, may so interfere with each other as to produce an unmelodious 

 rattling, or even silence. Also, two rays of light, whose paths bear a certain relation 

 to one another, instead of increasing each other's intensity, may have a directly oppo- 

 site effect, and, neutralizing each other, produce darkness. It becomes, therefore, a 

 question, not only of mere curiosity, but one whose bearings are important, to find if 

 the chemical rays emitted from the sun, when placed under similar circumstances, 

 exhibit similar phenomena. For then analogy would lead us to know that it is possible 

 for two rays ot HEAT to be so situated with regard to one another, as, instead of ex- 

 alting the temperature of the body on which they fell, to lower it ; or, hi other words, 

 to produce actual cold. 



447. In my early trials for the solution of this question I met with many disappoint- 

 ments, but at last I fell upon an arrangement which yielded positive information. It is, 

 however, an experiment requiring careful manipulation. A horizontal beam of light being 

 projected into a room by the apparatus heretofore so often referred to, at the extremity, e e 

 (Jig.69), of the brass tube, a double convex lens of short focus was screwed; this brought 

 the rays to a point at a distance of three quarters of an inch from the lens : here 

 they were obstructed by a metallic screen, b b, having a round hole, c, one eighth of an 

 inch in diameter, perforated in it. This screen revolved about a vertical axis on a pil- 

 lar, d, so that it could be brought to any angle with the incident rays. The rays passing 

 through the round hole, c, were received on a white screen, g g, at a distance of six 

 inches. When the screen b b received the incident rays perpendicularly to its surface. 



