426 Dr. Leeson on the Circular Polarization of Liglit 



To return, however, to the means by which the direction of 

 the rotation is to be ascertained. The method ah'eady described 

 might be applied in examining solutions of weak rotating energy 

 which exhibit little or no colour when examined by polarized 

 light, •provided it is not confounded mth the effects cf mere 

 depolarization^ the appearances then presented being similar 

 in some respects to those produced by double refracting sub- 

 stances, as described in the twenty-first chapter of Brewster's 

 Treatise on Optics. When substances of greater energy are 

 examined, they exhibit a gradation of colours corresponding 

 to those of the solar spectrum, which affords a still more sa- 

 tisfactory mode of determining whether the rotation is right- 

 or left-handed. The direction corresponds to that in which 

 tiie analyser must be moved in order to obtain the succession of 

 colours as follows, viz. red, orange, yellow, green, blue, indigo, 

 violet. If the analyser be moved in only one direction, and 

 that right-handed, or in the order of the figures in the outer 

 circle of graduation, then the rotation will be right- or left- 

 handed, according to the succession of the colours in the fol- 

 lowing tables. The general order of succession when the 

 analyser is moved in a n^'-Zi/- handed direction is first given, viz. 



The following table gives a more detailed account of the 

 succession of tints produced, and commences, it will be ob- 

 served, with the boundary of the blue and violet, which is the 

 darkest ray, corresponding to the greatest obscuration of X, 

 and is the point at which the degree of rotation should be ob- 

 served. We may derive much assistance in estimating its 

 precise position, when a double refracting analyser is em- 

 ployed, by examining the appearance of O, which will then 

 be at its maximum of illumination and of the brightest and 

 purest yellow tint. When we travel too far into the violet 

 with X, O will assume a greenish-yellow tint; when too far 

 into the blue, the yellow will deepen towards an orange. 

 If a tourmaline or coloured polarizer be employed, attention 



