PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 43 



as applied to the case in which the grooved face was turned towards the polarizer, is to be 

 understood the angle measured in air, from which the angle of diffraction within the glass may 

 be calculated from a knowledge of the refractive index. 



The grating being placed perpendicularly to the incident light, with the grooved face 

 towards the polarizer, the light diffracted at a considerable angle, (59° 52' by measurement,) to 

 the left of the regularly transmitted light was nearly white. When the pointer of the polarizer 

 stood at 70°, so that -zsr = + 45°, on turning the Nicol's prism of the analyzer in the positive 

 direction through the position of minimum illumination, the light became in succession greenish 

 yellow, blue, plum colour, nearly red. When sr was equal to — 45°, the same appearance 

 was presented on reversing the direction of rotation. Since the colours appeared in the order 

 blue, red, when tst = + 45°, and in the order red, blue, when w = - 45°, the analyzer being in 

 both cases supposed to turn in the direction of the hands of a watch, the deficiency of colour 

 took place in the order red, blue, when w m + 45°, and in the order blue, red, when w = - 45°. 

 Hence the planes of polarization, or approximate polarization, of the blue were more crowded 

 towards the plane of diffraction than those of the red. 



On placing a narrow slit so as to allow a small portion only of the diffracted light to pass, 

 and decomposing the light by a prism, in the manner already described, so as to get a spectrum 

 consisting of bright bands with dark intervals, and then analyzing this spectrum with a Nicol's 

 prism, it was found that at a moderate angle of diffraction all the colours were sensibly plane- 

 polarized, though the planes of polarization did not quite coincide. At a large angle of 

 diffraction the bright part of the spectrum did not quite disappear on turning round the Nicol's 

 prism, while the red and blue ends, probably on account of their less intensity, appeared to be 

 still perfectly plane-polarized, though not quite in the same plane. On treating in the same 

 manner the diffracted light produced when the grooved face of the glass plate was turned from 

 the polarizer, all the colours appeared to be sensibly plane-polarized. In the former case the 

 light of the brightest part of the spectrum was made to disappear, or nearly so, by using a thin 

 plate of mica in combination with the Nicol's prism, which shews that the defect of plane 

 polarization was due to a slight elliptic polarization. 



The numerical results of the experiments on the rotation of the plane of polarization are 

 contained in the following table. In this table ST is the reading of the polarizer corrected 

 for the index error 25°. A reading such as 340° is entered indifferently in the column headed 

 " tst" as +315° or - 45°, that is, 340° - 25° or - (360° - 340°) - 25°. a is the reading of the 

 analyzer, determined by one or more observations. The analyzer was graduated only from 

 - 90° to + 90°, and any reading such as - 20° is entered indifferently as - 20°, + l60°, or + 340°, 

 being entered in such a manner as to avoid breaking the sequence of the numbers. On account 

 of the light left at the minimum, the determination of a was very uncertain when the angle of 

 diffraction was large, except when tst had very nearly one of the values 0°, 90°, 380°, or 270°. 

 In the most favourable circumstances the mean error in the determination of a was about a 

 quarter of a degree. In some of the experiments a red glass was used to assist in rendering 

 the observations more definite. This had the advantage of stopping all rays except the red, 

 but the disadvantage of considerably diminishing the intensity of the light. The minutes in 

 the given value of 9, the angle of diffraction, cannot be trusted ; in fact, during any experi- 



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