152 



ACCESSORY APPARATUS. 



FIG. 51. 

 B 



is so turned that the side represented as undermost in the figure, 

 shall be uppermost ; and it is in order that high powers may be 

 used in this case as in the other, that the disk which then covers 

 the object is made of thin glass, instead of being (as in the 

 original form of the instrument) a piece of thick glass plate. 

 That a thin glass disk is more liable to fracture under pressure, 

 than a thick one, is no serious objection to its use for this pur- 

 pose ; since the lower one is not more likely to break than the 

 upper one ; and either may be replaced with extreme facility, 

 by simply warming the part of the instrument to which it is 

 attached, so as to loosen the cement that holds it. And the 

 advantage of being able to view an object under a high 

 power, from either side, will be most fully appreciated by 

 every one who has been much engaged in the 

 class of observations, which this instrument is 

 specially adapted to facilitate. If this Compres- 

 sorium be made of sufficient size to admit an or- 

 dinary glass slide between the vertical pins, an 

 object may be subjected to compression, and 

 afterwards removed for examination out of the 

 compressor, without transferring it from one glass 

 to another, which is frequently an advantage. 

 In this case, it will be convenient that the thin 

 glass disk should be "countersunk" into the upper 

 surface of the principal plate, so as to form one 

 level with it. 



71. Dipping Tubes. In every operation in 

 which small quantities of liquid, or small ob- 

 jects contained in liquid, have to be dealt 

 with by the Microscopist, he will find it a very 

 great convenience to be provided with a set of 

 tubes of the forms represented in Fig. 51, but of 

 somewhat larger dimensions. These were for- 

 merly designated as " fishing tubes;" the purpose 

 for which they were originally devised having 

 been the fishing out of Water-Fleas, aquatic 

 Insect-Larvae, the larger Animalcules, or other 

 living objects distinguishable either by the un- 

 aided eye or by the assistance of a magnifying 

 glass, from the vessels that may contain them. 

 But they are equally applicable, of course, to the 

 selection of minute Plants ; and they may be 

 turned to many other no less useful purposes, 

 some of which will be specified hereafter. When 

 it is desired to secure an object which can be seen 

 either with the eye alone or with a magnifying 

 glass, one of these tubes is passed down into the liquid, its upper 

 orifice having been previously closed by the forefinger, until 

 its lower orifice is close above the object; the finger being then 

 removed, the liquid suddenly rises into the tube, probably carrying 



Fishing Tubes. 



