ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



489 



The first, which he calls the protractor apertometer (fig. 101), con- 

 sists 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 of 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 



Fig. 101. 



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

 aperture in the silver film becomes coincident with the optic axis of the 

 Microscope ; then, on removing the eye-piece and looking down the tube, 

 the whole of that part of the protractor which the objective takes in will 

 be found to be visible, and the pointers can be adjusted so as to touch the 

 opposite edges of the field, the reading indicating the angular aperture. 

 Except with very low powers, however, the image of the protractor as 

 seen when looking down the tube is so small that some difficulty will 

 be experienced in adjusting the pointers. To obviate this the draw- 

 tube can be converted into an auxiliary Microscope, thus magnifying the 

 original image. This is effected by replacing the eye-piece and screw- 

 ing into the lower end of the draw-tube a very low-power objective, such 

 as the posterior half of a 2-in. objective. Having thus obtained a read- 

 ing of the actual angle embraced by the objective, it can be converted 

 to N.A. by reference to some such table as that in L>allinger's Carpenter. 

 The second simple form is the substage-scaJe apertometer, which was 

 August 20th, 1902 2 k 



