42 PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 



been reduced in the manner which will be explained in the next section, appeared somewhat 

 greater than was to have been expected from the first two experiments. This led me to 

 suspect that the crowding in the manner of reflection produced by diffraction accompanying 

 the passage of light from air, across the grooved surface, into the glass plate, might be greater 

 than the crowding had proved to be which was produced by diffraction accompanying the 

 passage from glass, across the grooved surface, into air. I accordingly placed the grating with 

 its plane perpendicular to the incident light, and the grooved face towards the polarizer, and 

 placed the analyzer so as to receive the light which was diffracted in passing across the first 

 surface, and then regularly refracted at the second. I soon found that the planes of polarization 

 were very decidedly crowded towards the plane of diffraction, and that, notwithstanding the 

 crowding in the contrary direction which must have been produced by the regular refraction 

 at the second surface of the plate, and the crowding, likewise in the contrary direction, which 

 might naturally be expected to result from the irregular refraction at the first surface, con- 

 sidered apart from diffraction. This result seemed to remove all doubt respecting the 

 hypothesis as to the direction of vibration to which the experiments pointed as the true one. 



On account of the decisive character of the result just mentioned, I took several sets of 

 observations on light diffracted in this manner at different angles. I also made two more 

 careful experiments of the same nature as the first two. The result now obtained was, that 

 there was a very sensible crowding towards the plane of diffraction when the grooved face was 

 turned from the polarizer, although there was evidently a marked difference between the two 

 cases, the crowding being much less than when the grooved face was turned towards the 

 polarizer. Even the first two experiments, now that I was aware of the index error of the 

 polarizer, appeared to indicate a small crowding in the same direction. 



Before giving the numerical results of the experiments, it may be as well to mention what 

 was observed respecting the defect of polarization. I would here remark that an investigation 

 of the precise nature of the diffracted light was beside the main object of my experiments, and 

 only a few observations were taken which belong to such an investigation. In what follows, 

 ■sr denotes the inclination of the plane of polarization of the light incident on the grating to a 

 vertical plane passing through the ray, that is, to a plane perpendicular to the plane of dif- 

 fraction. It is given by the reading of the pointer of the polarizer corrected for the index 

 error 25°, and is measured positive in the direction of revolution of the hands of a watch 

 placed with its back towards the incident light. 



Whether the diffraction accompanied reflection or refraction, external or internal, the 

 diffracted light was perfectly plane-polarized when tst had any one of the values 0°, 90°, 180°, 

 or 270°. The defect of polarization was greatest about 45° from any of the above positions. 

 When the diffracted light observed was red or reddish, on analyzation a blue light was seen 

 at or near the minimum ; when the diffracted light was blue or blueish, a red light was seen 

 at or near the minimum. When the angle of diffraction was moderately small, such as 15° or 

 20°, the defect of polarization was small or insensible ; when the angle of diffraction was large, 

 such as 50° or 60°, the defect of polarization was considerable. For equal angles of diffraction, 

 the defect of polarization was much greater when the grooved face was turned towards the 

 polarizer than when it was turned in the contrary direction. By the term angle of diffraction, 



