176 



Sir J. Conroy. On the Polarisation of [Feb. 4> 



experienced from this cause, as the little projections of plaster which 

 filled these holes effectually prevented any movement. 



In order to adjust the reflecting surface, a diaphragm with a small 

 hole in it was fitted into the eye end of the observing tube, and a lamp 

 with a flat wick so placed that its image was seen by reflection from 

 the surface of the crystal, two of the adjusting screws of the tube 

 being in the horizontal plane (i.e., the plane of incidence). The 

 tube was then turned through an angle of 180°, and the reflected 

 image brought back half way into the centre of the field by altering 

 the two screws of the tube which were in the plane of incidence, and 

 the remainder of the distance by means of the screws of the stage, 

 which, as well as the observing tube, remained clamped to the 

 horizontal circle whilst this adjustment was being made. 



The tube was then turned back to its original position, and the 

 adjustment repeated if necessary ; the tube was then turned through 

 90°, and the second pair Of screws altered till the reflected image 

 remained in the centre of the field whilst the crystal was rotated. 

 The adjustment was then examined by means of a simple form of 

 diagonal eye-piece placed in the collimator tube of the goniometer, 

 consisting of a brass tube with a diaphragm at either end, with a 

 small aperture in each, and also in the side of the tube ; inside the 

 tube, and opposite the aperture in the side, a piece of microscopical 

 cover-glass was fixed at an angle of 45° with the axis of the tube ; the 

 lamp was placed opposite the aperture in the side of the tube, and 

 the vertical stage rotated until the light of the lamp reflected from 

 the thin glass was reflected back by the crystal along the axis of the 

 eye-piece to the observer ; the tube holding the crystal was then 

 rotated, and if the spot of light remained visible whilst the tube made 

 a complete rotation, the adjustment was considered to have been 

 correctly made, and the position of the stage was then read on the 

 horizontal circle of the goniometer, and this measurement taken as 

 that of perpendicular incidence. 



Several such readings were made, and then the position of the tube 

 and lamp altered, and several more readings made, and the mean of 

 these, which usually were close together, taken as the zero for the 

 angle of incidence. 



Two complete series of observations were made with cleavage- 

 faces of Iceland spar in air, water, and tetrachloride of carbon, the 

 water and tetrachloride of carbon being contained by a nearly cylin- 

 drical thin glass vessel (a chemical beaker), which stood on the hori- 

 zontal stage of the goniometer, the tetrachloride being prevented from 

 evaporating by a layer of water floating on its surface. 



When the reflection took place in air, a paraffin lamp, with a flat 

 burner placed edgeways (i.e., radially to the goniometer) was used as 

 the source of light ; when the crystal was in water or tetrachloride 



