1897.J MICROSCOPICAL JOURNAL. 5 



to be examined, beginning with tlie lowest angled ones 

 and proceeding n})wards. 



With the tube length corresponding to the correction 

 of the objective if it is non-adjustable, focus the objec- 

 tive to be examined on the upper surface of the conden- 

 ser. Pressing the body tube against the rack to prevent 

 alteration of the focus, unscrew draw tube adapter and 

 remove draw tube. Into the lower end of the draw tube 

 screw a 3-inch objective. Replace draw tube in proper 

 position. This forms the auxiliary, observation or draw 

 tube microscope, and is for observing an image at its focal 

 distance through the objective under observation as a 

 medium admitting divei-gent rays of light, and not as an 

 objective. 



Reduce the aperture in iris diaphram of substage to 

 lowest size. Pressing body tube against rack as before 

 to prevent alteration of focus, focus the draw tube by 

 sliding it in the main tube sharply on opening in iris 

 diaphram Then open diaphram until only a glimpse of 

 its margin can be seen. The diameter of the opening 

 thus obtained is in direct ratio to the angular aperture of 

 the objective. Leaving the diaphragm as it is, repeat 

 the experiment using the next higher objective at hand, 

 remembering in every instance to remove the draw tube 

 objective and focus the one to be examined on the top 

 surface of condenser. In the second instance, after the 

 draw tube microscope has been focussed on the diaphram, 

 a margin will remain. Increase opening as before until 

 only a line of the margin of diaphragm is visible. 



The 'experiment may be repeated on higher powers un- 

 til the angle of aperture of the condenser system is 

 reached or approximated. 



While any great alteration in the focal distance of ob- 

 jective under observation will cause an appreciable error 

 in the^ comparison, a considerable range is allowable with- 

 out perceptible difference. Thus the experiment may be 

 much simplified and yet retain its accuracy by making 



