86 A. A. C. ELIOT MERLIN OK NEW DIATOM STRUCTURE. 



of the lens. In many cases, however, even on diatoms, where 

 minute details happen to be sufficiently contrasted, a deep 

 eye-piece will be found both serviceable and necessary, and the 

 wisdom of employing objectives of high optical index is manifest, 

 for their magnifying power can be easily varied by means of the 

 eye-piece to suit different objects. 



With regard to this question of magnification, I have prepared 

 dark-ground photographs of two different forms of Triceratium 

 novae zealandicae, x 490 (although both specimens are so named 

 by Moller, one example is double the size of the other). These 

 photographs were taken with a Zeiss 16-mm. apochromatic 

 objective, of measured N.A. 0*35 (really a 4-in., its measured 

 initial magnifying power on the 10-in. tube being x 18), and a 

 Powell 26-projection ocular. The focus was in both cases 

 adjusted on the images of the sub-stage apochromatic condenser 

 stop formed in the centre areolations, and the single bar arm of 

 the central stop is plainly pictured in many areolations of both 

 specimens. I venture to think that you may agree with me that 

 the scale of the photographs, for the aperture used, is by no 

 means too great when this bar is the object sought for, and that 

 there is really no useless over-magnification. 



Doubtless the bar could be detected on a smaller-scale picture 

 when its existence is once known ; but if it were not especially 

 sought for, the chances of accidentally overlooking it would be 

 greatly increased were less power employed. Now, the 1-in. 

 apochromat of N.A. 0*32 would require the 40 eye-piece to 

 visually yield a magnification of 400 — a ratio of less pow r er to 

 aperture than that of the photographs. The negatives were 

 taken with the nominal 16 mm., instead of the 24 mm., for the 

 reason that sufficient camera extension w T as not available to 

 obtain the required enlargement with the latter lens, even with 

 a 26-projection ocular. I should add that, visually, the stop and 

 bar images appeared sharper and blacker than in the photo- 

 graphs, which w r ere secured by means of a ^-in. lamp-flame and 

 Nelson's apian atic doublet auxiliary condenser. No screen was 

 employed, as even without one an exposure of an hour for each 

 plate proved barely sufficient. 



Journ. Quekett Microscopical Club. Ser. 2, Vol. X., No. 60, April 190T 



