304 S. C. AKEHURST OX SOME OBSERVATIONS CONCERNING 



and when the mirror was slightly tilted, if the diatom was a 

 suitable one. it was resolved into dots. A good image of the 

 rosettes on Coscinodiscus asteromphcdus was obtained, which 

 stood a high-power eve-piece well. With the mirror slightly 

 tilted, the faintly marked transverse striae were visible on Cymato- 

 pleura solea also an excellent black-dot image was displayed of 

 Xo.ck'da rhomboid.es, SurireUa gemma and Pleurosigma angu- 

 IcUuin. On examining a strewn slide of Xacicula r~homboid.es 

 in realgar I found a specimen of Pinnularia nobilisl On tilting 

 the mirror and obtaining oblique light the costae were filled with 

 dots. Particulars of this were forwarded to Mr. Nelson, who 

 replied as follows: "Mr. Merlin and I have seen the structure 

 on Pinnularia to which you refer. It was demonstrated upwards 

 of twenty years ago by H. Gill, who tilled up the apertures in 

 diatoms with platinum some of these specimens I have still." I 

 am pleased to be able to give this report, as it helps to dispose of 

 the idea that might arise that the dots displayed were probably 

 due to false images, brought about by using annular light. 



The opaque lines on an Abbe test plate were well defined, and 

 an excellent rendering of stained bacteria, such as Tubercle bacilli,, 

 was obtained. In all the tests referred to the following combina- 

 tion was used : Incandescent gaslight, Nelson stand condenser, 

 Leitz concentric reflecting condenser and tiuorite, T Vth inch oil- 

 immersiun objective N.A. 1*35, Wiukel complanat eye-piece, and 

 Wratten B screen. 



During the autumn of 1913 Mr. O'Donohoe became interested 



in this reflecting condenser, and he spent an evening with me 



examining some of the test objects referred to ; and afterwards 



kindly undertook to see if any results worth attention could be 



obtained by photography when using this type of condenser. He 



was successful in getting a record of the dots on Pinnularia.* 



I am very much indebted to Mr. O'Donohoe for the ready 

 manner in which he undertook the work of testing the condenser, 



* T. A. O'Donohoe : "An Attempt to Resolve Pinnularia xobilis,'' p. 309. 



