JOURNAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



DECEMBER, 1918. 



TRANSACTIONS OF THE SOCIETY. 



XII. — An Improved Method of Apertometry. 



By H. Hartkidge, M.A.. M.D., F.R.M.S., Fellow of ICing's 



College, Cambridge. 



{Read December 18, 1918.) 

 FouB Text-Figs. 



Inaccukacy in apertometry is found to be due to the following 

 causes : — 



1. The use of inaccurate focus or incorrect tube length. 



2. The use of rays not strictly corresponding to the centre 



of the field. 



3. The incorrect setting of the indices. 



4. The incorrect determination of the position of the indices 



in relationship to the graduated scale ; and inaccuracy 

 in the graduated scale itself. 



5. Employment of an incorrect angle between the plane of 



the apertometer plate and the optical axis. 



6. Chromatic aberration. 



7. The employment of methods based on the measurement 



of the Ramsden disc. 



Section 1. — Error in Apertometry due to the Use of 

 Inaccurate Focus or Incorrect Tube Length. 



The graduation of the scale of the Abbe apertometer is carried 

 out on the supposition that the segments illuminated by rays 

 proceeding to the periphery of the disc have their apices in 



2 A 



