1886.] Light by Reflection from Iceland Spar. 175 



Professor Stokes very kindly called my attention to these experi- 

 ments of Sir David Brewster, and pointed out that as they had never 

 been published in detail, and had not been repeated by anyone else, 

 it was desirable that further observations should be made on this 

 subject. The experiments, the results of which I have the honour of 

 submitting to the Royal Society, were undertaken at Professor Stokes' 

 suggestion, and in carrying them out I had the benefit of his advice. 



The apparatus used was essentially the same as that employed by 

 Seebeck ; the divided circle of the goniometer was, however, horizontal, 

 and not vertical, as in Seebeck's instrument, and the arrangement for 

 keeping the reflected ray constantly in the axis of the observing tube, 

 whilst the angle of incidence was varied, differed from that employed 

 by him ; the axes of the stage and of the observing tube were fitted 

 with toothed wheels which geared into a double pinion, the diameters 

 of the wheels being such that the angular velocity of the observing 

 tube was double that of the stage. 



The goniometer had a vertical stage in addition to the ordinary 

 horizontal one, which could be moved nearer to and further from 

 the axis of the instrument, and this stage had four adjusting screws, so 

 that the front surface could be placed parallel to the axis of rotation. 



A brass plate was clamped to this stage ; a short brass tube carrying 

 an annular toothed wheel at one end, and a divided collar at the other, 

 fitted into an aperture at the lower end of this plate, and could be 

 rotated in the plane of the plate by turning a milled head fixed at the 

 end of a rod, which carried a bevel pinion working into the annular 

 wheel. 



A brass tube with a collar at one end could be fastened to the 

 annular wheel by four screws passing through holes in the collar, the 

 back of the collar and the inner surface of the wheel being portions of 

 a spherical surface. 



The crystal whose reflective power was to be examined was fixed in 

 the inner tube, which was then adjusted by means of the four screws 

 so that the surface of the crystal was in the plane of rotation, and 

 then, by altering the position of the vertical stage, the plane of rota- 

 tion brought vertically over the axis of the goniometer. 



The crystal was cemented into the tube with plaster of Paris ; a 

 little lard was rubbed on the edges of the face which was to be exposed, 

 and it was placed on a plate of glass with this surface downwards, 

 and the brass tube, the collar of which had also been greased, placed 

 round it, and centred by means of a marked card placed under the 

 glass, and plaster of Paris poured in. In one of the earlier experi- 

 ments the crystal was found after a certain number of observations 

 had been made to have become loose, owing to the plaster having 

 shrunk away from the tube ; three holes about 2 mm. in diameter were 

 therefore drilled in the sides of the tube, and no further difficulty was 



