156 



NOTES. 



Photography of Diatoms. 

 By T. A. O'Donohoe. 



Plate VII. 



At the January Meeting I exhibited three photographs of Pleuro- 

 dynia angulatum, and one each of Surirella gemma (black dots), 

 Amphiplcura pellucida, Coscinodiscus aster omphahis, and Podura 

 scale. These were all taken direct at an amplification of 2000 

 diameters, except the Amphiplcura pellucida, which was magnified 

 1500 times. Two photographs of the Pleurosigma angulatum 

 were further enlarged to 3700 times. 



The illumination was furnished by an ordinary oil lamp with 

 | in. wick. Watson's parachromatic condenser was used to pro- 

 cure direct axial cones, except in the case of the Amphiplcura 

 pellucida, for which Watson's immersion condenser N.A. 1*30 was 

 used to obtain the necessary oblique illumination. 



The exposures ranged from 2 hours without a screen, as in the 

 case of the Pleurosigma angulatum herewith reproduced, to 6 hours 

 with the F line screen, as in the case of the Podura scale also re- 

 produced. These long exposures of course necessitate a very good 

 fine-adjustment in the stand. Mine is the portable histological stand 

 made by Swift for and exhibited by Mr. E. M. Nelson at the meet- 

 ing of the E.M.S., held December 18, 1895. 



It is fitted with Campbell's differential screw. The lens used 

 is a Zeiss apochromatic 2 mm. N. A. 1 * 30 with a Zeiss 4 com- 

 pensating ocular, as my 2 projection ocular would not give the 

 required amplification. 



I use an enlarging camera, costing a few shillings, extending 

 to 40 inches and sliding on a home-made optical (?) bench fitted 

 with a home-made fine-adjustment, which answers well enough for 

 low-power work, but which was detached altogether in taking the 

 photographs now under consideration. 



I focused in the air with an achromatic hand reading-glass, 

 and passed backward and forward, touching the fine- adjustment of 

 the Microscope until the desired result was obtained. This pro- 

 cess requires patience and perseverance — two excellent qualities 

 to acquire. 



For the Podura scale and Coscinodiscus aster omphalus I used an 

 axial cone of 0*5; for the Pleurosigma angulatum, * 65 ; for the 



