PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 49 



calculated from (49) with the assumed value of log m, and is put down for the sake of com- 

 parison with the value of a — iff deduced from the difference, a — w, of the observed angles a, 

 •&, corrected for the assumed index error e. The upper and lower angles belonging to any pair 

 found in the second and third columns refer respectively to the upper and lower signs in the 

 first column, and in the column headed "ar". In the table, the experiments are arranged in 

 classes, according to their nature, and those belonging to the same class are arranged according 

 to the values of 9. The first three experiments in the table relate to diffraction at refraction, 

 in which the grooved face of the grating was turned from the polarizer, the next six to diffrac- 

 tion at refraction, in which the grooved face was turned towards the polarizer, and the last two 

 to the experiments in which the grating was a little inclined, and the three principal images 

 were observed. The result of Experiment No 1, is here given separately, on account of the 

 different values of iff there employed. 



Experiment No 1. 6 = 57°5'; assumed index error e = 40°5'. 



W a — •& 



- 115° - 1°46' 



- 92^° - 0°3l' 



- 70° 



- 47£° + 0°33' 



- 25° - 0°14' 



- 2£> + 0°16' 

 + 20° + 1° 



+ 42l° + 0°19' 



+ 65° + l°4l' 



The values of a for w = - 115° and 73- = + 65° ought to differ by 180°, whereas they differ by 

 3°27' more. This angle is so large compared with the angles a - •ar given just above, that it 

 seems best to reject the experiment. The experiment is sufficient however to shew that the 

 crowding of the planes of polarization, be it in what direction it may, is very small. On com- 

 bining all the observations belonging to this experiment in the manner already described, a 

 small positive value of log rn, namely + .002, appeared to result. This value, if exact, would 

 indicate an extremely small crowding in the manner of reflection. 



Vol. IX. Pakt I. 



