336 N. E. BROWN ON THE STRUCTURE OF DIATOMS. 



the distance to which the pseudopodia (if I may term them so) 

 extended was between 1/3, 000th and 1/4, 000th of an inch. After 

 carrying it along across about one-third of the field of view, 

 it released its hold of the dirt, and in doing so I saw it give a 

 very slight but distinct jerk, just as if something had snapped 

 suddenly, for the mass of dirt was very much larger than itself. 

 This observation was made with a fth lens. 



Finally a word as to the pores. It must not be expected that 

 they can be rendered visible in as easy a manner as Surirella 

 gemma can be resolved into dots, for they cannot ; they are so 

 extremely minute that they are by no means easy to detect. 

 To make them out at all a Tjyth. or xV^ n oil-immersion of 

 1ST. A. 1*3 is necessary, with eye-pieces of sufficient power to bring 

 the magnification up to at least 1,000 diameters, and often not 

 less than 2,000 diameters is really required to make the structure 

 clear, combined with very careful manipulation, a most exact 

 arrangement of the light and a fair stock of patience. Some can 

 be seen with central light, but for the most parti have found that 

 the easiest way to render them visible is by means of a Leitz 

 dark-ground illuminator, from which, by decentring it, various 

 modifications of oblique light and light reflected from the under 

 surface of the cover-glass can be obtained. This method of 

 reflecting light upon a diatom from the under surface of the 

 cover-glass may not be generally known, but it can be accom- 

 plished by decentring the condenser or dark-ground illuminator, 

 and then raising or lowering it slightly until the right effect is 

 produced. The process is not a difficult operation, but requires a 

 little practice, and very often features can be seen much more 

 clearly by this method than by any other. It is like viewing an 

 object upon which the sun is shining, with the back to the sun. 

 When examining a diatom by means of the Leitz illuminator no 

 funnel-stop must be used in the lens to cut down its aperture. 

 Sometimes a rather dim light is better than a bright one for 

 rendering the structure conspicuous. 



The lowest power with which I have been able to see the pores 

 in the films of Pleurosigma balticum is Powell & Lealand's 

 excellent |-th water-immersion, with which, in combination with 

 a X 18 eye-piece, they are just perceptible. A Leitz ygth or 

 yg-th oil-immersion will also demonstrate them and those of 

 other species, but the lens I have chiefly used has been a Iteichert 



