APPENDICES 807 



Number of Times of Reduction. 



2 34 5 6 



Fccus of Lens. Distance in Inches of Lens from Focussing-screen. 



3 inches. 



4 - 



6 12 9 8 -74- 7^ 



9 



V. THE DETERMINATION OF THE N.A. OF MICROSCOPE 

 OBJECTIVES 



While an expert observer is often able to estimate approxi- 

 mately the N.A. of a microscope objective by direct observation 

 of some known specimen, yet in ordinary practice some more 

 exact method of determination is necessary. There are 

 two appliances available, either of which is suitable for use 

 with both dry- and oil-immersion lenses ; one is known as the 

 Abbe Apertometer, and the other as Cheshire's Apertometer. 

 There are other methods in use which are only applicable to 

 dry lenses. 



The Abbe appliance (Fig. 78) consists of a segment of 

 glass about three inches in diameter and half an inch thick, 

 the segrnental portion of which cc is bevelled from above 

 downwards and towards the semicircular edge to an angle 

 of 45. Near the middle of the straight edge a small disc 

 of silvered glass a is affixed, from the centre of which a small 

 circle of the silvering has been removed. Two sliding metal 

 plates bb are so fitted on the semicircular edge that they can 

 be moved round to any position. This edge of the plate is 

 also graduated and marked with the full range of apertures 

 up to 1'4, which represents practically the highest N.A. which 

 is likely to be measured. To use the appliance the microscope 

 is placed in a vertical position, and the glass plate is placed 

 on the stage with the graduated surface uppermost. The 

 objective, the N.A. of which is to be measured, is placed on 

 the microscope and the hole in the centre of the small glass 

 disc is carefully focussed. An eye-piece which has a large 

 field should be placed in position, the general conditions under 

 which the objective is to be used, such as the correct tube- 

 length, &c., being duly observed. 



x2 



