1 42 PHOTO-MICROGRAPHY 



Another disturbing appearance is that of a distinct iraage of the dots being occa- 

 sionally seen outside the diatom itself, lying in space ! Here we are confronted with 

 an exhibition that nothing but optical phenonriena can possibly explain. It is for these 

 reasons we never employ oblique light unless it emphasises what can he seen ivithout 

 its use. If by its extreme use we benefit our pictures, but get these " outside " effects, 

 we reject them, and only accept photographs which, it is true, may not to the eye be 

 so elegant, or to the photo -micrographer so excellent, but to the mind which prefers 

 unquestionable reality, will appeal all the more. 



General Remarks. — In taking such objects as blood-cells, eosinophyl-cells, and the 

 like — Fig. 7, Plate IV. — the operator will have to use a somewhat contracted cone of 

 light, so as to bring out the definition of the edges of the principal objects, and he may 

 have to use a touch of the Davis diaphragm to increase the definition of the little dots 

 in the eosinophyl cells if his objective be not a very good one. The exposure when 

 using the green pot glass will be about half a minute. Development must not be 

 carried too far, or the little dots before referred to will be clogged up. 



It should be remarked that the difiiculty with all diatoms is to get a sufiiciently 

 dense negative, especially when using high 7yiagniJicatio7is. This trouble always arises 

 from mc/er-exposure. When developing, the operator must not think his negative over- 

 exposed because it appears to " come up " quickly. The rapid development of the 

 image will always be present in diatoms because it is obvious the whole plate is 

 exposed to almost the same amount of hglit ; the only interception to it being the 

 thin markings of the diatom itself 



Mem oranda. — W^ith green screens, say with the Giffbrd F line, the exposure has 

 usually to be augmented about three times ; with a red screen and a Lumiere red- 

 stained plate about four times ; with these employ amidol for a developer, as Hydro- 

 kinone fogs them. With yellow screens about twice the exposure is sufficient — 

 Edwards plate ; whilst with violet screens and Edwards plate hardly any increase is 

 required. Occasionally it may be necessary to gain sufficient density to use an 

 Edwards " Snapshot " plate. It is four times quicker than the " Medium." Develop 

 deeply. Do not have quite so much soda in developer. 



In comparing the photographs taken by different lenses the same N. A. should be 

 utilised if a faithful comparison is to be made. 



All immersion lenses work quicker than dry ones : for example, a dry sixth works 

 slower than an immersion xVth with the same amplification. 



Some diatoms are easier to photograph than others. The Navicula spectabilis is a 

 fairly easy one and often selected to show how achromatics compare with apochromatics, 



