TECHNOLOGY OF DIATOMS. 349 



the greater the ease of the operator. As to the eye-piece a No. 3 

 ought to be sufficient, otherwise the eye will be too quickly tired/ 

 the rays reflected from the mirror of the microscope will give 

 light enough ; an iris diaphragm is useful for moderating the light. 

 When the light is too intense, I use a piece of clear blue glass 

 fixed under the stage of the instrument, to soften it still more, the 

 work of selection being trying to the eyes, especially when it is 

 continued for many consecutive hours. 



In a combination of this kind, there is yet one difficulty to 

 overcome — namely, the reversal of the image of the object. The 

 employment of a correcting eye-piece takes away too much light 

 and definition ; besides, it fatigues the eye more, so I advise our 

 readers to neglect this, for they will actually become, and that in a 

 very short time, so accustomed to this optical effect that they will 

 not notice it, or, at all events, that it will not inconvenience them. 



It is the same in the use of the mechanical finger, and of all 

 other contrivances more or less ingenious intended for picking up 

 Diatoms. In my opinion, the hand is the only sure instrument, 

 and in a relatively short time it is possible to acquire sufficient 

 steadiness. I recommend the use of very simple instruments. 

 The bristle of a living hog, or of one recently killed, or better still 

 a hair from the belly of a hedgehog, carefully chosen by means of 

 a pocket lens, fixed with wax to the end of a light pen-holder, 

 or held within the jaws of a light drawing-pen, constitutes the 

 instrument. 



I prefer using a drawing-pen to anything else, because then it 

 is easy to lengthen or shorten the hair according to the require- 

 ments of the occasion. You can hardly understand, when we have 

 got a really good bristle, how careful we are of it. 



I pass on to notice the little incidents that may happen during 

 the selection. 



The Diatoms jump and pass out of the field of vision on 

 contact with the bristle. 



There are two causes for this : — First, the distilled water has 

 not been perfectly pure, or possibly the Diatoms have not been 

 sufficiently washed ; in these cases the valves adhere more or less 

 to the glass, and the bristle, in suddenly detaching them, makes 



