194 



Sir J. Conroy. 



[Jan. 10, 



about 60° with the normal, no regularly reflected light could reach 

 the observer. 



The terminal faces of the polarising Nicol were perpendicular to its 

 long diameter, and it was of a nearly circular section, so that the 

 amount of light transmitted by the Nicol must have been very nearly, 

 if not absolutely, the same in both positions. 



Table IX gives observations made to determine the amount of 

 illumination in both cases, the numbers in the first column being the 

 distances at which the sliding lamp had to be placed when the light 

 was polarised in the plane of incidence, and those in the second, when 

 it was polarised in a plane at right angles to this ; each determination 

 being the mean of four observations. 



Table IX. 



70-9 85-3 



70-0 90-4 



68-8 . . 86 '9 



70-0 88-8 



Mean.... 69 '9 87 '8 



Calling the light diffused by the paper when the incident light was 

 polarised in the plane of incidence 100, it would appear that when the 

 light was polarised perpendicularly to that plane only about 63 per 

 cent, of the light reached the observer. 



The very considerable difference in the amount of light diffused in 

 the two cases seems the more remarkable, as previous to the publica- 

 tion of the former paper the illuminated surfaces of the photometer 

 were examined with a buquartz, and although they showed traces 

 of polarisation it was only, apparently at least, to a very inconsiderable 

 extent. 



This experiment has been recently repeated, and with the same 

 result ; the lamp light diffused by the paper of the photometer 

 showing hardly any, if any, traces of polarisation, whilst that reflected 

 obliquely from the blackened surface of the board along which the 

 lamp was arranged to slide, when examined in the same way, was seen 

 to be strongly polarised.* 



* [Of the light falling on the paper, a considerable part would be reflected at 

 various depths before it had lost, through the various irregular reflections and 

 refractions, all traces of its original polarisation, and consequently light derived from 

 that which was originally polarised in the plane of incidence would be more 

 copiously reflected than light derived from that which had been polarised in a per- 

 pendicular plane. But the light so reflected would have to make its way among 

 the fibres of the paper, especially as the angle of emergence was considerable, and 

 in so doing would be pretty well depolarised by tlie irregular reflections and 

 refractions which it would have to undergo. This accounts for the circumstance 



