•278 PROFESSOR POWELL ON THE ELLIPTIC POLARIZATION OF LIGHT 



lowered as required ; and should be capable of a slight inclination for better adjust- 

 ment. 



The middle part or hinge (M) is surrounded by a graduated vertical circle, by 

 which the parts (P) and (A) can be set at any angle to each other: they are attached 

 to it by projecting arms : the whole is moved by the joint (K) till the reflected light 

 from (R) comes distinctly to the eye through (E), when (it is easily seen) the angle 

 of incidence will be half that measured on (M). 



The zero of (M) is found by adjusting (P) and (A) with their axes in one line (or 

 reducing the instrument to an ordinary polariscope). The hinge should be capable 

 of being firmly clamped. 



In observations at very great incidences it will be easily seen to be necessary to 

 have the arms carrying (P) and (A) so fixed, that when the axes of the tubes are 

 brought into one line there shall be a considerable space between their inner ends : 

 there should be also a slight motion about the axis of the arm for adjustment in (P). 



In observations at very small incidences, it is most convenient to throw the light 

 on to (P) by means of a small mirror placed in a proper position. 



Fig. 2 represents the mode of applying the Fresnel rhomb (F) to the analyzing 

 part, the rest of the arrangement remaining the same. The eye-piece (E) is removed 

 to a cap on the top of the rhomb ; while the bottom of the rhomb is attached to a 

 short tube, which fits into the aperture before occupied by the eye-piece. 



Fig. 3 represents the appearance of the rings in elliptic light when the analyzer 

 is at 45° between the dark and light systems. 



Observations. 



The determination of the precise position of the analyzer at which the maximum 

 or minimum brightness is attainedj is from the nature of the case open to consider- 

 able uncertainty. But the intervention of the rings, instead of using the simple ana- 

 lyzer, affords some aid in this respect, since the change in the nature and form of 

 the rings offers a more ready guide to the eye ; especially in elliptic light, where there 

 is never a total evanescence. Still the undefined nature of the object observed pre- 

 cludes minute numerical accuracy, and the results in the following tables must be 

 regarded as no more than approximations, though derived from the means of a great 

 number of repetitions. In taking these means, I have omitted fractions of a degree 

 as bearing an appearance of accuracy quite illusory. 



The greatest care is necessary in the adjustments, especially of the polarizer. They 

 were remade for each set of observations. That for the polarizer to obtain the zero 

 or coincidence with the plane of reflexion, was found by varying the azimuth till the 

 light was perfectly restored to plane polarization ; the analyzer being in the position 

 for dark branches, the zero of its circle was determined at the same time. 



In the observations at different incidences, in order to secure the constancy of its 

 inclination to the plane of incidence, the polarizer was adjusted to zero before obser- 



