STUDY OF DIATOMS. 27 



either with the continental foot or a tripod ; the former is more adapted 

 for photographing difficult objects in a nearly vertical position, the latter is 

 more suitable and stable in the inclined position in which finished 

 preparations have to be examined. 



The stereoscopic binocular microscope, almost unknown on the con- 

 tinent and so important in histological research, is not indispensable for 

 observing diatoms. With high powers the Wenham Prism and High-p9wer 

 Prism are of no use, so also Abbe's stereoscopic apparatus, though fine detail 

 can still be seen with high-power objectives, fails nevertheless to give a 

 sharp image, which thus renders details less visible than with a monocular 

 microscope. 



The ordinary binocular, giving a moderate magnification {i.e.. 200 

 diameters at least), is however remarkably useful for resolving the often 

 complicated structure of Crypto-raphideas, and I venture to assert that it is 

 impossible to form so good an idea of a diatom as when it is observed 

 through a binocular microscope. 



The objectives which a diatomist requires are not numerous. Every 

 difficulty can be met with four objectives of graduated power. In 

 order to avoid any loss of time it is best that these objectives should be 

 attached to a well-constructed nose-piece permanently fitted to the micro- 

 scope. However the piece of apparatus invented by Dr. R. Zeiss called 

 a " Sliding objective changer," is even preferable, because it enables each 

 separate objective to be perfectly centered. 



The objectives which I now use for ordinary research are all apochro- 

 matic. They are the 8mm. (r-3rd inch), 3mm. dry (i-8th inch), and 

 2'5mm. (i-ioth inch) N.A. 1.25 water immersion. When they are 

 insufficient, e.g., for observing certain minute details, I have recourse to 

 the 3 - o or 2 - o mm. (i-Sth or i-i2th inch) homogeneous. But these cases 

 are rare, for they never occur when making ordinary observations for 

 determining species, but only when elucidating structure, when all the 

 resources of optics have to be utilised, and even Zeiss' famous objective of 

 N.A. 1 *6 is by no means too much. Nine-tenths of the observations for 

 determining species can be made with an 8mm. objective in combination 

 with a series of compensating eye-pieces, particularly No. 8 and 18. 

 We consider that this objective is the best that modern optics have produced 

 for all kinds of ordinary work. An experienced eye can easily, with its 

 aid, detect the bacillus tuberculosis. 



My work on the Microscope ( x ) affords all necessary information on the 

 subject. 



( T ) English Edition, London, 1893, pp. 101-102, 44-57, etc. 



