56 Dr. J. Sutherland on the Polarization 



spaces dark, with segments of two or three darker circles on 

 them. 



I have also used a section of rock crystal for the purpose 

 of obtaining impressions of its rings, and to determine whether 

 phaenomena were presented similar to those of circular polar- 

 ization. The result, however, owing to the unsettled state of 

 the weather, was not so satisfactory as I could have desired, 

 and I have therefore left this part of the subject for a more 

 favourable opportunity. 



The next step in the investigation was to determine whether 

 similar phaenomena were presented at the violet extremity of 

 the spectrum. For this purpose I employed a glass prism to 

 decompose the polarized sunbeam used in the preceding ex- 

 periments. A polarized spectrum was thus formed, at the 

 extreme violet end of which most of the experiments were re- 

 peated. The extreme violet ray was allowed to pass through 

 the prism of calcareous spar, and received on photogenic paper; 

 one image of violet light, extremely feeble, was all that was 

 visible. The action of the chemical ray was, however, intense, 

 for in a minute or two a deep dark spot marked the position 

 of the unextinguished ray, while no effect whatever was pro- 

 duced by the extinguished ray. A film of mica was now intro- 

 duced into the course of the violet ray ; two faint luminous 

 images appeared, and two dark impressions were obtained. 

 These experiments were again repeated beyond the extreme 

 violet ray. That part of the spectrum which traversed the 

 prism of calcareous spar gave no luminous image, but the re- 

 sult was the same ; to wit, the unextinguished chemical ray 

 gave a dark impression on sensitive paper, and the extin- 

 guished ray none; and when a film of mica was used, two 

 dark impressions of both rays were obtained. 



The experiments with the rings were also tried ; but al- 

 though the impressions were visible, they were by no means 

 so distinct as those obtained from the direct sun-light, a cir- 

 cumstance which is partly to be attributed to the great diffi- 

 culty of keeping the axis of the apparatus employed in the 

 axis of the polarized ray. The sun's motion has to be com- 

 pensated by the movement of the hand; and these experi- 

 ments are on this account of difficult performance, unless a 

 heliostate, or some similar contrivance, be used to keep the 

 sunbeam precisely in the same direction during the required 

 time. 



2. Polarization of the Chemical Rays by Pejlexion. 



I have now stated the principal results at which I have ar- 

 rived in the polarization of the chemical rays by double refrac- 



