56 PRACTICAL MICROSCOPY. 



Much has been said relative to the aperture of objec- 

 tives; therefore it may be of interest to the student to 

 know how the measurement of this angle may be set 

 about. Colonel Woodward and Mr. Wenham both prefer 

 modifications of Professor Robinson's method, as being the 

 most accurate. 



Colonel Woodward screws the objective to be tested into 

 a tube, which pierces the shutter of his dark room, the 

 back of the objective being towards the light. A parallel 

 pencil of sunlight is then thrown through it by means of a 

 solar mirror, and the cone of light is measured off directly 

 upon a cardboard protractor, arranged with its centre in the 

 focal point of the lens. 



Another method is to fix the microscope body so that it 

 may rotate upon a pin placed with the focal point of the 

 objective as a centre. This is then placed on a board 

 graduated into degrees, and the light of a candle, placed a 

 few feet off, viewed through the eye-piece. The body of 

 the microscope is turned until only half the field remains 

 illuminated, and the angle read off upon the graduated 

 circle below. Mr. Wenham declares this method faulty, 

 and also passes some strictures upon Colonel Woodward's 

 plan, stating that a border of diffused light exists which 

 renders the measurements inaccurate, the extreme angles 

 having often to be guessed at. 



Dr. G. E. Blackham, in his paper already quoted, gives a 

 method suitable for most objectives, a plan which can easily 

 be carried out with one of the stands fitted with a swinging 

 substage. A plano-convex lens is employed, of such thick- 

 ness that when the plane side is connected with the under 

 surface of the object-slide with softened balsam, the thick- 

 ness of slide, balsam, and lens shall equal the radius of 

 curvature. This lens is connected by balsam to the under 

 side of the slide containing a balsam-mounted object. 



