Optical Test for Angles of Contact. 287 



collimator with its axis horizontal and with a horizontal slit 

 be placed to the left o£ the drop, parallel light will fall per- 

 pendicular to the glass plate as shown in the figure, and the 

 concave lens will form an image of the slit, which will be a 

 straight line through perpendicular to the plane of the 

 paper, and in the same horizontal plane as the corresponding 

 line through 0. If now a low-power microscope be placed 

 to the right of the drop with its axis along OA, the image of 

 the slit formed at A can be seen and arranged to coincide 

 with the horizontal cross-wire of the microscope. If the 

 microscope be now moved back through a distance AO it 

 will be focussed on the glass plate, and the image in the 

 microscope of the upper edge of the drop should either 

 coincide with the horizontal cross-wire of the microscope or 

 it should be necessary to raise the microscope vertically to 

 make it do so. 



If an angle of contact existed, the drop would present the 

 appearance of fig. 1 (b). The continuation of one side of 

 the drop has been dotted in to the point P, where it would 

 become vertical ; and it will be observed that in this case 

 the centre of the lens which is in the same horizontal plane 

 as P is above the upper edge of the drop. Thus after 

 focussing the microscope on A, getting the image to coincide 

 with the horizontal cross-wire, and then moving it back 

 through the distance AO so as to focus on the glass plate, the 

 microscope must be lowered a distance ON to get the image 

 of the upper edge of the drop to coincide with the cross-wire. 

 The microscope used was the Bailey & Smith microscope 

 used in the previous experiment (loc. cit.). This was capable 

 of horizontal and vertical motions, and was arranged with 

 axis along the line of parallel light and its horizontal motion 

 along this line. A clean microscope slide-cover was used as 

 the glass plate and the liquids examined were those of the 

 former experiment — water, glycerine, olive-oil, and turpentine. 

 In all of these when the drop was freshly formed a perfectly 

 straight image of the slit was seen, and to get the image of 

 the upper -edge of the drop (sometimes marked by a line 

 of extremely minute air-bubbles) to coincide with the cross- 

 wire when the microscope was brought back the microscope 

 had to be raised. The image of this edge was not straight. 

 As evaporation went on the upper edge came down to the 

 same level as A (which of course also moved, both vertically 

 and in a horizontal direction perpendicular to the plate); and 

 as evaporation proceeded further the image became curved 

 and took the shape of the upper edge of the drop. JSTo indi- 

 cation of the state of things represented in fig. 1 (b) could be 



