An Improved Method of Apertometry. 



343 



A better plan has been suggested to me by Ainslie, namely, to 

 utilize the principle of tlie diagonal scale. For this purpose a 

 specially graduated glass plate would be required, as shown in 

 fig. 3. The method of using such a scale is well known, and there- 

 fore needs no further description. The reason for the use of curved 

 lines is similar to that <^iven above. 



Fig. 3. — Diagram to show the design of an apertometer with a 

 diagonal scale, thus giving direct readings to O'Ol N.A. 



Section 5. — Error in Apertometry due to Plane of Aperto- 

 MKTER Plate not being at Eight* Angles to the 

 Optical Axis of the Microscope. 



Since the light leaves the objective to form a cone of illumina- 

 tion, and since a cone has its greatest width across its diameter, it 

 is clear that the pointers of the indices must be symmetrically 

 placed on a true diameter to the cone, for otherwise the aperture 

 reading will be too low. Since the prism angle at the back of the 

 plate is 45° it might be thtnight that this point could be neglected. 

 Tests show that this is not the case. Without considering the 

 possible causes of error, the following method of obtaining the 

 correct adjustment may be described as follows : — If with the 

 objective focused in the cover slip of the apertometer, the indices 

 appear to be out of line with the diameter of the cone, then the 

 apertometer plate must be tilted so as to bring them as far as 

 possible correctly into line. The fine centring is now performed 

 by sliding the plate as a whole forward or backward. (This 

 method of adjustment is impossible with the old type of Abbe 

 apertometer, in which the silvering forms a complete ring under 

 the cover glass ; and therefore adjustment must be effected by 

 tilting alone.) 



