ACCESSORY INSTRUMENTS. 133 



most of the animalcules with it ; in this case, it is by far the 

 best plan to wipe away all the fluid from the bottom-plate 

 and the channel, and make the latter and the under surface of 

 the thin glass cover perfectly dry before another drop is put 

 upon the bottom glass, otherwise the channel, when once 

 made wet, will attract the fluid again. In the animalcule 

 cages, or live boxes, manufactured by Mr. Pritchard, the 

 bottom plate of glass is ruled with fine lines, the one-hun- 

 dredth part of an inch or less apart, to serve as a micrometer. 

 When used dry, the lines are visible, but when fluid is inter- 

 posed, they can not only hardly be seen, but all measurements 

 made by such micrometers are manifestly incorrect with 

 objects of any degree of thickness, as their true outline is not 

 in focus at the same time as the lines of the micrometer ; this 

 point will be particularly dwelt upon in the chapter devoted 

 to the measurement of objects, but in this place it merely 

 requires to be noticed in connection with the instrument to 

 which it is applied. 



Fishing Tubes for Animalcules. — These consist of tubes of 

 glass, about nine inches in length, open at both ends, and from 

 one eighth to one-fourth of an inch in diameter ; the ends 

 should be nicely rounded off in the flame of the blow-pipe; some 

 of them may be straight, as shown by A, fig. 86, whilst others 

 should be drawn out to a fine point, as C, or curved as B, D ; 

 in short, they may be made of either of the shapes represented 

 in fig. 86, all of which have been found exceedingly useful. 

 Mr. Varley, to whom we are indebted for this valuable inven- 

 tion, describes the method of using them in vol. forty-eight of 

 the Transactions of the Society of Arts. Supposing the ani- 

 malcules about to be examined to be contained in a 

 phial or glass jar, as in fig. 87 ; having observed where they 

 are most numerous, either with the naked eye if they are 

 large, or with a pocket magnifier or the watchmaker's lens 

 described at page 50 if they are small ; either of the glass 

 tubes, having one end previously closed by the thumb or fore- 

 finger wetted for the purpose, is introduced into the phial in 

 the manner represented by the figure ; this prevents the water 

 from entering the tube, and when the end is near to the 



