308 S. C. AKEHURST ON SUB-STAGE ILLUMINATION. 



Both these pictures were taken by the same man, using the 

 same objective, diatom and illumination the only difference 

 being in the condenser used. Regarding this matter, Mr. 

 O'Donohoe writes as follows : " I was never able to see the 

 black-dot image when using my ordinary oil-immersion condenser, 

 hence was much surprised to find that the reflecting concentric 

 condenser showed the black dots beautifully. This and the 

 Amphipleura show that the reflecting condenser is a better re- 

 solver than my ordinary oil-immersion condenser and axial 

 illumination. 



Fig. 3. Amphipleura pellucida x 2,000. This photograph was 

 taken to demonstrate the usefulness of annular light when 

 searching a slide for fine structure. The diatoms are at right 

 angles to each other, and both resolved. Had light in one 

 azimuth been employed, such as one gets with an achromatic 

 condenser, and quarter-moon stop, only one would have been 

 resolved, viz. the diatom with striae at right angles to the 

 direction of the beam of light. 



Fig. 4. A record of Surirella gemma x 2,000. This was 

 taken with the reflecting condenser. The black dot is shown, 

 and at the same time the ribs are resolved into dots. 



Plate 22. 



Fig. 1. Navicida rkomboides x 1,500, taken with the re- 

 flecting condenser. 



Fig. 2. Pinnidaria nobdis x 2,500, showing the costae filled 

 with dots. Taken with the reflecting condenser. 



Journ. Quekett Microscopical Club, Scr. 2, Vol. XII., No. 74, April 1914. 



