810 APPENDICES 



The Cheshire apertometer (Ffg. 79) is somewhat more 

 simple in use, but it does not admit of such exact determinations 

 with high apertures. It consists of a circular disc of glass 

 with a mark on its upper surface, on which the objective 

 to be measured is carefully focussed. The lower surface of 

 the apertometer is ruled with a number of concentric rings, the 

 distance between each corresponding to O'l N.A. In use 

 the apertometer is placed on the stage of the microscope and 

 the cross lines ruled on the surface are brought into focus, the 

 eye-piece is then removed and by direct observation the 

 number of concentric circles in view can be counted : this 

 gives directly the N.A. of the objective. 



With high powers it is difficult to count the lines without 

 some accessory observing appliance, since they appear so close 

 together. A special eye-piece which focusses to the back 

 focal plane of the objective may be obtained, and this is 

 substituted for the ordinary eye-piece. The reading is then 

 taken through the special eye-piece as before, and the aperture 

 is indicated by the number of concentric circles that are visible. 



VI. BIBLIOGRAPHY 



1853. On the Application of Photography to the Repre- 

 sentation of Microscopic Objects. J. Delves. 

 Transactions of the Microscopical Society of London. 

 (The first published account of any work on Photo- 

 micrography.) 



BOOKS OF REFERENCE ON MICROSCOPY AND ALLIED SUBJECTS 



DR. W. B. CARPENTER. The Microscope and its Revelations. 

 (Edited by the late Dr. Dallinger.) 



LEWIS WRIGHT. Popular Handbook to the Microscope. 



W. BAGSHAW. Elementary Photo-micrography. 



CONRAD BECK. Cantor Lectures on the Theory of the 

 Microscope. Royal Society of Arts. (One of the most 

 lucid expositions of the subject yet written.) 



DR. E. C. BOUSFIELD. Guide to the Science of Photo- 

 micrography. 



A. C. MALLEY. Micro-Photography. Wet Collodion Process. 



A. PRINGLE. Practical Photo-micrography. 



F. SHILLINGTON SCALES. Practical Microscopy. 



