308 APPENDICES 



With the apertometer an additional objective is supplied. 

 The draw-tube of the microscope is now removed, and this 

 objective is screwed into the bottom of the draw-tube, the 

 eye-piece and the original lens being still allowed to remain 

 in position. The draw-tube is now returned to the body of 

 the microscope, and is focussed down until the images of 

 the two metal plates which slide round over the indices can 

 be seen, these being moved round until they are in the field 

 of view. It is important to see when returning the body- 

 tube to the microscope that no alteration in the position of 

 the objective which is being measured takes place. It must 

 still remain in the position to which it was adjusted, to show 

 with an ocular, and without the additional objective on the 



c c 



FIG. 78. Apertometer, No. 155. (Full size.) 



inside of the draw-tube, an image of the small spot in the 

 centre of the silvered glass disc. The sliding metal pieces 

 are carefully moved round until just their edges show on 

 either side of the luminous disc as seen in the eye-piece. The 

 reading of the indices may now be taken by direct observation, 

 and if the objectives and eye-piece are all exactly central and 

 in alignment it will be found that the reading is exactly the 

 same on both sides, and that the reading represents the exact 

 N.A. of the objective. It may happen that some slight 

 difference in the reading on either side results, and in this 

 case the mean of the two readings may be taken, the result 

 given being the exact N.A. The observing ocular must 

 have a sufficiently large field to enable the whole of the field 

 of view to be directly observed. 



The makers give the following further instructions : 



