HIGH-POWER OK CRITICAL PHOTO -MICROGRAPHY 135 



will look aa if it was steeped in mud. The N. A. ro condenser noiist be used, and we 

 confess to have a predilection for Powell & Lealand's new apochromatic, Conrady's 

 N.A. -95, or the new Parachromatic by Watson. If we use Powell's, as it is not 

 quite free from spherical aberration, it must be centred with the greatest of care. 

 First, then, close the iris as small as possible, lower the condenser till the image of 

 the hole is seen with a low eye-piece, centre it and re-obtain critical light. Expose, 

 about five to ten seconds ; it is short, but the object is exceedingly faint. Slightly 

 close the iris with caution and open the Davis diaphragm. 



Navicula spectabilis, Fig. 12, Plate Y., may also be photographed with the 

 ■J- objective, but we prefer the |-. Keep the diaphragm rather more closed and Avidely 

 open the Davis; the exposure about the same. This specimen is not easy to do 

 because of the thickness of the structure. If the iris be too much closed, ditfraction 

 effects will spoil the final result. 



We proceed now to a more difficult, in point of fact the most difficult part of the 

 subject. We refer to the use of the ^ and immersions. We must premise our re- 

 marks upon the use of these lenses by saying that as the ^ and rV apochromatics each 

 have a N. A. of i "40, so nothing save magnification is gained by one power over the 

 other. With respect purely to our own personal experience we find that for general 

 work, excepting that of photographing bacteria, no lens ever made, that we have seen, 

 and we have tested a good many in the last twenty years, can compare with the 

 3 mm. apochromatic made by Zeiss. A large working distance, excellent definition, 

 and a perfection of optical centreing are all combined in this lens. For photographing 

 diatoms we know no equal. It will stand a 27 eye- piece with suitable objects without 

 producing the faintest falling-off in any respect ! But for bacteria, we do not find the 

 field is so flat as we could desire, and the by Powell & Lealand, especially made 

 for this work, whilst not rendering such good performance with diatoms, is far better 

 for the purpose. The field is so much flatter in the Pov/ell lens, whilst the definition 

 of the faintest flagellum is superb. Hence for diatoms we always employ the Zeiss 

 3 mm., and for bacteria the Powell & Lealand. It must be distinctly understood 

 here by the reader that nothing invidious is meant by the above remarks, and in 

 stating our experience it is only given for what it is worth with the objectives in our 

 actual possession and may not be in accord with that of others. 



With these high powers it is needless to point out that only a very small portion of 

 a specimen can be photographed at one given time ; hence it has to be carefully 

 searched for a typical place in case of tissues, bacilli, &c., and for a well-marked and 

 evenly mounted specimen when dealing with diatoms. It is best to do this at some 



