422 Dr. Leeson on the Circular Polarization of Light 



On using as an analyser a rhomb of Iceland spar, or a prism 

 of quartz constructed as before alluded to, to examine a ray 

 of polarized light transmitted through a small circular aper- 

 ture, the oppositely polarized beams of light, O and X, may 

 be observed together, and they will become alternately light 

 and dark as the analyser is revolved, according to the law al- 

 ready laid down. The ordinary ray being represented by O, 

 and what is termed the extraordinary ray by X, the rhomb 

 or prism should be so arranged or posited in its setting as 

 that O be central, and therefore apparently stationary, whilst 

 X revolves around O*, as the analyser is itself rotated. Each 

 portion, O and X, returns to its original condition after a 

 revolution of half a circle, or 180°, and it passes from the 

 state of greatest obscuration to that of maximum illumina- 

 tion on a rotation of 90°, or quarter of a revolution. 



In all these cases, then, of what is termed rectangular po- 

 larization, the maximum of illumination takes place when the 

 analyser is placed with its plane of polarization corresponding 

 to that of the ray to be transmitted, whilst such ray attains 

 its greatest obscuration when the analyser is placed at right 

 angles to such plane. Now this condition is altered when a 

 medium of a certain thickness fj possessing what is termed 

 circular polarization, is introduced between the polarizer and 

 analyser; for if the polarizer and analyser be placed so that 

 one ray, X, is at its greatest obscuration or invisible, and the 

 other, O, at its maximum of illumination, then on introducing 



in the extent of the field of view, which is a great advantage in examining 

 large specimens, such as unannealed glass, although not so important in 

 circular polarization. 



I may here observe, that the sky at all times furnishes polarized light, 

 which is more or less intense according to the relative position of the ob- 

 server and the sun. This circumstance probably influences the chemical 

 action of the rays of light, and points out the necessity of varying the posi- 

 tion of the camera, or the period of exposure in taking Daguerreotypes 

 and other optical investigations, according to the time of day, the state 

 of the sky, and the sun's declination. Any person may readily ascertain 

 where the greatest intensity of polarization exists by using a tourmaline, 

 or other analysing medium, and looking through it at a piece of selenite, 

 or a piece of Iceland spar or other crystal, cut to exhibit the cross and 

 rings; of course the colours will be most vivid, and the black cross most 

 intense, where the greatest polarizing force exists. This polarization is 

 most intense where the sky is blue and unclouded. The planes of pola- 

 rization appear to vary at a total distance of 90°, or of 4o° measured each 

 way from the sun's position. 



* This is very important to prevent confusion arising from a revolution 

 of the two images around each other, whereby the one may be mistaken 

 for the other. 



t A certain thickness is here stated, because, as will be hereafter shown, 

 it is possible in every case to employ a thickness which would produce no 

 apparent alteration. 



