4f^8 Dr. Leeson on-ihe Circular Polarization ofLighi 



occurs four times in the course of a complete revolution of 

 360°, viz. twice in the ordinary, and twice in the extraordinary 

 ray. With a tourmaline or a Nicol's prism, the tints will of 

 course occur only twice in a whole revolution. 



That in a revolution of 180° the tints have passed through 

 all their phases. 



The tints that have been described correspond to those of 

 quartz, and are more or less vivid in proportion to the rota- 

 ting energy. In fluids the blue violet generally assumes more 

 of a neutral tint, and in many instances the crimson is scarcely 

 to be observed. Of course, if the fluid be itself possessed of 

 colour, it will greatly interfere with the purity of the tints. 

 When using a piece of red glass, as recommended by Biot, to 

 obtain homogeneous light, the tints disappear, and the ob- 

 scuration of X, which is then more complete, will determine 

 the amount of rotation. The red glass sent by Mons. Soleil 

 with Biot's apparatus is much too dark, and occasions so great 

 a loss of light as materially to interfere with the delicacy of our 

 readings. Of course the direction of the rotation should be 

 observed ^without it. 



Daylight is preferable to artificial light for these experi- 

 ments ; unless, as before explained, in the case of a coloured 

 polarizer, attention is paid to the appearance of O when arti- 

 ficial light is employed, the results will not agree with those 

 obtained by daylight. 



Having explained the appearances which are to guide our 

 experiments, I proceed to a description of the apparatus 

 which I constructed for the purpose, and which is similar in 

 some respects to the arrangement of the Rev. B. Powell, de- 

 scribed in the Philosophical Magazine for April 1843. 



M N O P (figs. 1 and 4, p. 429) is a wooden box, the lid of 

 which h h not only opens in front, but may be also fixed 

 at any height, and fastened by a small bolt at «, fig. 1. This 

 box serves the purpose of a darkened chamber, and the ap- 

 paratus may be packed in it when not in use. K K is a 

 moveable perforated shelf, which may be also secured at any 

 height by means of the screw at I, fig. 1, passing through an 

 opening in the back of the box shown at X, fig. 4. On 

 this shelf the vessel containing the fluid to be examined is to be 

 placed, its centrality being provided for by fitting into another 

 loose shelf turned out to suit it. To the under side of the shelf 

 K a brass plate is screwed, fitted with a socket to receive the 

 polarizer G, which may be either a tourmaline or a Nicol's 

 prism, and which may be revolved by means of the collar at- 

 tached to it for that purpose. H is a mirror to reflect the 

 light through the polarizer and liquid towards the analysing 



