54 Dr. J. Sutherland on the Polarization 



careous spar, one and three quarters of" an inch in length and 

 one inch in the side, and polished at each end, was employed. 

 It was inclosed in a case having an aperture at one extremity, 

 the other extremity being open. The extreme violet rays of 

 the solar spectrum were allowed to pass through the aperture 

 and to fall on a piece of photogenic paper. Two very faint 

 images were formed, and in a minute or two these produced 

 corresponding dark impressions on the paper. A similar re- 

 sult was also obtained when the direct sun's rays were em- 

 ployed. By this apparatus two impressions of equal intensity, 

 each half an inch long and one-eighth of an inch broad, were 

 obtained; but on extending the experiments I soon found 

 that a polarized beam of greater size than it could give was 

 necessary ; I therefore substituted a plate of Iceland spar an 

 inch square and ^ths of an inch thick, and in order to in- 

 crease the divergence of the rays, one of the planes was ground 

 to an angle of 63° with the obtuse edge, and both planes were 

 then polished. A plate of Iceland spar thus prepared, has 

 the property of separating the two rays so much, that when 

 inserted into an aperture admitting a sun-beam into a dark- 

 ened room, it gives two images of polarized light, each one 

 inch in diameter and about an inch apart, on a screen placed 

 at the distance of eight feet from the aperture. 



These images, when received on sensitive paper, both pro- 

 duced considerable effect ; but the extraordinary more than 

 the ordinary, and it was therefore chosen for the purpose of 

 experiment. 



An analyzing apparatus, consisting of six thin plates of 

 mica, was placed obliquely in the course of the polarized ray, 

 so as to form with its axis an angle of about 25°. The in- 

 strument was turned round until the plane of the mica plates 

 coincided with the plane of polarization of the ray. When this 

 was done the light was almost extinguished, and was allowed 

 to fall on a piece of photogenic paper. After the lapse of five 

 minutes no effect whatever was produced on the paper. The 

 mica plates were then turned round 90°, until their plane was 

 at right angles with the plane of polarization. The light was 

 greatly increased in intensity, and in one minute the paper 

 was tinged, in three minutes a good deal so, and in five mi- 

 nutes it was pretty dark. This experiment proves that the 

 plane of polarization of the chemical rays is coincident with 

 that of the luminous rays of the sun's light. 



Instead of the mica plates employed in the last experiment, 

 I next used the long prism of Iceland spar already mentioned. 

 The polarized beam was transmitted along it, and the prism 

 turned on its axis until one of the rays was extinguished : a 



