210 



H. F. ANGUS ON THE APERTOMETER AND ITS USE. 



and the latter can be obtained for a few shillings, thus disposing 

 of the first objection. Before considering the second it would, 

 perhaps, be well to give a detailed description of these two pieces 

 of apparatus and their use. 



The Protractor a2)ertometer, as will be seen from the figure, 

 consists essentially of an ordinary semicircular protractor 

 mounted on a base, and supplied with two pointers, easily 



adjustable to indicate any angle, and an object placed at the 

 centre on which to focus the objective under examination.. In 

 practice, a silvered cover-glass mounted on a piece of glass the 

 thickness of an ordinary slip, with an aperture of about 1 mm., is 

 found the best object on which to focus. The objective is focussed 

 in the usual manner on the edge of the aperture in the silvered 

 surface ; the protractor is then moved slightly, so that the edge on 

 which the focus was obtained moves out of the field, and the 



