8 F. J. CHESHIRE ON ABBE's TEST OF APLANATISM. 



read as centimetres. Should the apertometer, when made, be 

 too large to be accommodated on the stage of the microscope, 

 with its centre in the axis of the instrument, it should be 

 cut down on one side until it is possible to do so. 



In using this apertometer it is necessary to observe the image in 

 the upper focal plane of the objective, either directly, or after it 

 has been magnified in some way. Whatever method is adopted it 

 is important that a small stop should be used, placed virtually at 

 the point p, to sharply define the apex of the cone of light taken 

 up by the objective. One of the following methods may be 

 employed : — 



1. The unassisted eye may be used, in which case the image 

 of the eye-pupil formed by the objective serves to define the 

 point p. Care should be taken to keep the eye fixed during the 

 taking of a reading. 



2. The observation may be made through a 2 — 3 mm. hole in a 

 plate on the top of the draw-tube, replacing the ordinary eye- 

 piece. 



3. The bottom of the draw- tube may be fitted with a low- 

 power lens, with a small stop near its upper focal plane — this 

 lens forming with an eye-piece a low-power telescope. 



4. By using a low-power eye-piece — the lower the better — 

 fitted with a 2 — 3 mm. stop in the usual place, to form in the eye- 

 ring an image of the image in the upper focal plane of the 

 objective. The eye-ring may then be examined with a hand- 

 magnifier. 



The 2nd and 4th methods will generally be found the most 

 convenient — the first for testing low-power objectives, and the 

 second for testing high-power ones. 



[The diagrams on the loose plate accompanying this paper 

 are intended to be cut out and used on the microscope in the 

 way described. The apertometer disc should be placed upon the 

 microscope stage, with its centre in the axis of the instrument, 

 upon which the objective to be tested should be focussed. Then, 

 upon racking back the body through 25 mm., and removing the 

 eye-piece, the N.A. of the objective will be found projected in 

 its upper focal plane.] 



Jovrn. Quekett Microscopical Club, Ser. 2, Vol. IX., No. 54, April 1904. 



