of the Chemical Rays of Light. 55 



piece of sensitive paper received a dark image from the unex- 

 tinguished ray ; but the extinguished ray produced no effect 

 whatever. 



A film of mica was then placed in the course of the polar- 

 ized beam before it passed through the prism, and the ex- 

 tinguished ray immediately reappeared : the two rays were 

 allowed to fall on sensitive paper, and both produced tints of 

 equal intensity. 



This experiment was repeated with a film of selenite in- 

 stead of the mica : one of the rays was coloured of a yellowish, 

 the other of a purple tint : on being received on photogenic 

 paper both the images gave dark impressions, but the purple 

 image produced more effect than the other. 



The experiment was again repeated with a film of selenite, 

 which gave a pink colour to one ray and a green colour to 

 the other ; and in this instance both images gave tints of equal 

 depth to the paper. These experiments go to prove that the 

 chemical rays, when polarized, are acted upon by thin cry- 

 stallized plates, in a manner similar to that in which the lu- 

 minous rays are influenced. 



I was next desirous of ascertaining whether any phaenomena 

 resembling the coloured rings seen round the axes of crystals 

 in polarized light were presented by the chemical rays when 

 polarized, and for this purpose I employed an apparatus con- 

 sisting of a tube two inches long and three-fourths of an inch 

 in diameter : at one extremity of it was placed a double con- 

 vex lens, having a focus of one and a quarter inch : within the 

 tube, and at the distance of half an inch from the lens, was 

 placed a section of a calcareous spar rhomb, such as is used 

 for showing the coloured rings. At the other extremity of 

 the tube was placed an oblique analyzing bundle of three mica 

 plates, or one of Nicol's improved prisms, and the appara- 

 tus was so disposed that the polarized sunbeam was allowed 

 to fall on the lens, and thence through the tube upon a screen 

 placed close to it. An image of the coloured rings and black 

 cross was thus obtained, and by turning the tube 90° upon 

 its axis, the rings with the white cross appeared ; while in this 

 position a piece of photogenic paper was used to receive the 

 image, and a reversed impression of the rings and cross was 

 obtained ; to wit, the place where the white cross had been 

 was dark, the centre light, with a complete black ring round 

 it, and segments of other rings exterior to it. The tube was 

 next turned 90° upon its axis, so as to show an image of the 

 rings with the black cross; sensitive paper was again em- 

 ployed, and another reversed impression obtained; to wit, 

 the position of the black cross was white, the centre and inter- 



