494 DUNCAN J. REID ON CRITICAL ILLUMINATION IN 



and free from spots, which may otherwise show on the field. In 

 using non-critical illumination the iris of both condenser and 

 collecting lens must be fully open, or nearly so. 



Personally I prefer critical illummation, which when using 

 a collecting lens is capable of giving us full N.A. ; this the 

 non-critical illumination may not do, and the former is easy to 

 regulate by opening or closing the iris of the condenser. If any 

 spots or imperfections on the collecting lens show on the field, I 

 rack the condenser very slightly outside or inside the focus. 



7. Size of Apertui^e Employed Working Aperture. 



As I have already said, few lenses give perfect definition with 

 critical illumination at full aperture. The question then arises 

 to what extent should it be cut down. Mr. Nelson, I understand, 

 says that a 3/4th cone is usually the one which gives the best 

 results. Some measurements which T have made tend, however, 

 to show that the largest percentage aperture with which perfect 

 definition can be obtained depends not only on the lens itself, 

 but on the object, on the medium in which it is mounted, on 

 the condenser and also on the system of illumination. 



The best method of deciding when the aperture has been 

 sufficiently cut down is to keep one's eye on the field and 

 gradually close the iris until all flooding with light has dis- 

 appeared. 



Some objects such as blood films, bacteria, etc., will often stand 

 the full aperture, whilst diatoms may require it to be closed con- 

 siderably. Dry objects do not stand such a large aperture as 

 those in balsam. The more perfect also the correction of the 

 conden.ser, the larger the aperture that can be used without 

 " flooding." 



8. General Method of Arranging Light and Apparatus for High, 



Medium and Low Poivers. 



Having arranged the microscope at a convenient angle for 

 work (with, say a- 4 mm. or l/6th objective, and fairly low 

 ocular, a blood film on the stage, and a substage condenser of 

 rO N.A.) the lamp (with wick trimmed to give a well-shaped 

 flame) should be placed about a foot away and directly in front, 

 where it is more easily adjusted than at the side, and raised so 



