188 



Sir J. Conroy. 



[Jan. 10, 



mirrors with polarised light. The polish of the tin and silver mirrors 

 being defective, it was not thought worth while to re-examine them. 



The general arrangement of the apparatus remained the same ; but; 

 in order to obtain a more intense light, a magic lantern (the one 

 known as the " Sciopticon " being used) was substituted for the 

 paraffine lamp carried by the goniometer. It was fixed on a wood 

 stage fastened to a stout board about 80 centims. long; a screw, 

 which passed through the board near one end and was fixed in a 

 table, formed an axis about which the board could rotate ; the gonio- 

 meter was fixed to the board with its centre vertically over the axis 

 of rotation. 



A black card, with a slit 54 millims. by 4 millims., was placed in 

 the slide-holder of the lantern, and a Nicol in the collimator tube of 

 the goniometer, and the image of the slit iocussed on the paper of 

 the photometer. 



The metal plates were clamped to the vertical stage, and their 

 adjustment examined by placing a second, or analysing, Nicol in the 

 path of the reflected light and crossing the Nicols, the former being 

 placed with its principal section either in or perpendicular to the 

 plane of incidence, and adjusting the stage screws till the light re- 

 flected from the plate was completely extinguished. 



The experiments were made in the manner described in the former 

 paper, the light being polarised in, or perpendicularly to, the plane of 

 incidence by the Nicol. It was found that the illumination of the paper 

 varied with the position of the Nicol, being always greatest when the 

 light w 7 hich fell on the paper was polarised in the plane of incidence. 



Table I gives a series of observations made with the steel plate 

 with light polarised in the plane of incidence. The numbers in the 

 first column are the distances, in centimetres, of the sliding lamp from 

 the photometer when the light from the lantern fell directly on the 

 paper; and those in the third when the light was reflected by the 

 mirror. The means of these observations are contained in the second 

 and fourth columns, the angles of incidence in the fifth, and the 

 ratios of the reflected to the incident light — the latter being taken as 

 100 — in the sixth colamn. 



As the intensity of light varies inversely as the square of the dis- 

 tance from the source, the percentage reflected by the plate is obtained 

 by dividing the numbers contained in the second column by those in 

 the fourth, squaring, and multiplying by 100. 



Table II gives a similar series of measurements with the same 

 plate and light polarised perpendicularly to the plane of incidence. 



Three other similar series of observations were made, the actual 

 determinations being about as concordant as those contained in the 

 tables. The four sets of observations, and their means, are given in 

 Tables III and IV. 



