428 Mr. Watson's Experiment with the Solar Microscope. [Dec. 



Article IV. 



Experiment with the Solar Microscope. By Mr. James Watson. 



(To Dr. Thomson.) 



SIR, London, Sept. 23, 1819. 



A few years ago, while engaged in making some experiments 

 on the prismatic colours, I was agreeably surprised by the sudden 

 appearance of a new aud interesting picture. In beauty it far 

 excelled the common spectrum. It rather resembled a cloud, 

 composed of the finest colours ; and having a peculiar veined or 

 clou led appearance, which added variety and richness to this 

 exquisite picture. I have frequently repeated the experiments, 

 with additional improvements, from time to time, to the gieat 

 pleasure of several friends who have seen it. As some of your 

 readers may wish to judge for themselves, I shall now beg to 

 communicate the following particulars through the medium of 

 your journal. 



Any person having a solar microscope will only require in 

 addition a glass prism and a pair of tin plate tubes, about two 

 inches long, and about the same in diameter, each fitted with a 

 convex lens at one end. These tubes may be found ready 

 prepared in a middle sized magic lantern. 



1 shall now describe an easy and convenient method of 

 mounting the prism and tubes. Procure two pieces of mahogany, 

 one piece 18 inches long, 5 inches broad, and 1 inch thick ; the 

 other 4 inches square, and 2 inches thick. Take the long piece 

 of mahogany, and at three inches from one end, on the flat side, 

 cut a shallow hole, just large enough for one end of the glass 

 prism to turn in. A piece of stout brass wire is then to be stuck 

 into the wood, at two inches from the same end. If then this 

 wire be bent over at the top into the form of a ring, it will sup- 

 port the upper end of the prism, and hold it in a perpendicular 

 position, admitting also of its being turned round. This being 

 finished, take some of the same brass wire, and make two rings 

 with stems to them, each ring just large enough to receive one 

 of the tin tubes. Then bore a couple of holes, 24- inches apart, 

 in the square piece of mahogany ; place the stems of the wire 

 rings in these holes, and see that the rings do not stand higher 

 than the upper end of the prism. The tubes are now to be 

 placed in the rings with the glasses towards each other ; their 

 distance can be adjusted by moving them backwards or forwards ; 

 and if the wires have room to turn in the holes, an horizontal 

 motion may be given to the tubes when necessary. 



The apparatus is now prepared for use. The best time for 



Eerforming the experiment is from spring to autumn, about the 

 our of noon, and when the sky is free from clouds. The room 

 is to be made as dark as possible, and the solar apparatus fixed 



