292 Lord Rayleigh. On the Intensity of [Nov. 18, 



During these observations a disk was employed giving trans- 

 mission 0*0577, so that in this case the final result is absolutely 

 independent of the effect of the adjustable oblique plate. It will be 

 observed that the separate results obtained by Mrs. Sidgwick and by 

 myself differ, even in the means, by 1 per cent. This is not the only 

 instance in which the errors have presented a suspiciously systematic 

 appearance ; but the differences being always small could not be sub- 

 mitted to any satisfactory examination. It rarely happened, for 

 instance, that Mrs. Sidgwick and I could find definite fault with 

 each other's settings. 



When these results were first obtained, I thought that they would 

 turn out to be too high for agreement with Fresnel's formula, suppos- 

 ing that the index of the glass was low. A subsequent measurement 

 of the specific gravity, however, gave reason for suspecting that the 

 glass might be flint, a conclusion confirmed by determinations of the 

 refractive index. 



These were made by two methods : (1) by observation of the 

 polarising angle in air, (2) by observation of the angle at which total 

 reflection sets in when the mirror is immersed in bisulphide of carbon. 

 The first is, perhaps, the simpler in respect of experimental arrange- 

 ments, but it is open to the objection that the inference of the 

 refractive index from the polarising angle is somewhat theoretical. 



The black glass was mounted upon the turntable of an ordinary 

 goniometer. In the focus of the collimator was placed a wire, seen 

 dark in a bright field of view. Various positions of the turntable 

 were then tried, such that on rotating a Nicol held at the eye the dark 

 patch appeared to pass somewhat to the right or to the left of the 

 collimator wire. After each observation the web of the telescope was 

 set to coincidence with the collimator wire, and a reading taken. 

 Success depends in some degree upon the use of a suitable light. 

 Sunshine diffused through ground glass answered the purpose very 

 well. 



Eight. 

 64° 25' 

 16 

 17 

 14 



The table gives a set of circle readings. In the first column the 

 patch was to the right of the collimator wire, in the second to the 

 left, and in the third there was no appreciable deviation. We may, 

 therefore, take as the reading for the polarising angle 64° 5', with a 

 probable error not exceeding 3' or 4'. The reading for a direct 

 setting of the telescope upon the collimator wire was —5', so that the 

 polarising angle is -J(180— 64° 10')= 5 7° 55'. Whence according to 

 Brewster's law — 



Left. 

 63° 56' 

 40 

 55 

 55 



Central. 



64° 7' 

 64 



