PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 45 



multiple of 90°. This rendered the observations very uncertain. About the octants the mean 

 error in a set of observations taken one immediately after another amounted to near 2°. 



No. 17. a determined by two observations. The light was very imperfectly polarized, 

 except near the standard points. Yet the observations agreed very fairly with one another. 

 The mean of the mean errors was 25', and the greatest of them not quite 1°. 



No. 18. a determined by two observations, which, generally speaking, agreed well with 

 one another. For •& = - 90° and W = + 225° the light observed was rather scattered than 

 regularly diffracted, the sheet of the cone of illumination having fallen above or below the hole 

 of the analyzer. 



No. 21. a determined by two observations at least. In this experiment the polarizer was 

 covered with red glass. 



No. 22. a determined by two observations. Marked in note book as " a very satisfactory 

 experiment, though the light was not perfectly polarized." 



No. 23. a determined by two observations at least. The hole in a screen placed between 

 the polarizer and the grating was covered with red glass. This appears to have been a good 

 experiment. 



No. 11. a determined by two observations, which agreed well with one another. In the 

 table, a (l), a (2), a (3) refer respectively to the first, second, and third of the three principal 

 images already mentioned. In this experiment the polarizer was reversed, that face being 

 turned towards the mirror which in the other experiments was turned towards the grating, which 

 is the reason why a and •zzr increase together, although the light observed suffered one reflection. 

 The same index error as before, namely 25°, is supposed to belong to the polarizer in its 

 reversed position. 



No. 12. ct determined by three observations. The largeness of the angle of diffraction 

 rendered the determination of a very uncertain. 



