STUDY OF DIATOMS. 25 



default of this, a mineral oil lamp can be used. An excellent little lamp of 

 this kind is supplied by Messrs. Watson and Sons for 16s. 



Instruments. Microscope. Excellent instruments are made in England, 

 Germany, France, and the United States ; but all these instruments are 

 not equally convenient for diatom work. An instrument will not be 

 suitable unless it is furnished with an excellent condenser, enabling us 

 to change from axial to ultra-oblique illumination instantly and without 

 difficulty. We know of two condensers which perfectly fulfil these 

 conditions, viz., Abbe's condenser and Powell and Lealand's oil condenser. 

 The first of these apparatus is in more general use than the second, and 

 satisfies all the requirements of the microscopist ; the second, made specially 

 for examining diatoms, enables the diatomist to work more rapidly and 

 with greater exactness ; the light having previously been arranged a lever 

 has only to be pressed in order to change immediately from axial to 

 any degree of oblique illumination. For some years Powell and 

 Lealand have constructed an optical system, the lower lens of which has 

 the middle part stopped, and the whole can be substituted for the ordinary 

 optical system. It is used to obtain extreme obliquity, and is useful, 

 for instance, for resolving the longitudinal striae of an Amphipleura. 



Achromatic condensers, especially for photo-micrography, are to be 

 preferred to those that are not achromatised. Of achromatic condensers 

 Powell and Lealand's apochromatic oil condenser is the best for resolving 

 diatoms, on account of the largeness of its aperture, which is nearly as 

 much as 1.4 N.A. The microscope of a diatomist who wishes to exhaustively 

 investigate the organisms which form the subject of this work should be 

 a first-class one ; it should be quite firm and its coarse and fine adjust- 

 ments as perfect as possible. 



A sliding movement, used as a coarse adjustment, would prevent the 

 correct centering of the condenser to be maintained ; the adjustment must 

 therefore be effected with a rack and pinion. 



It is an advantage for the stage of the microscope to be fitted with 

 a mechanical stage, by which diatoms can be easily found again and the 

 entire preparation easily examined with the certainty that nothing has been 

 lost sight of. 



The large and medium sized microscopes of Zeiss are excellent, when 

 furnished with Abbe condensers ; but we always prefer to use the large 

 English forms made by Ross and Powell and Lealand. The Van Heurck 

 microscope, which is constructed by Messrs. Watson and Sons according 

 to our specifications, realizes all that a diatomist can desire, for both 

 the purposes of observation and photo-micrography. It is constructed 



