48 SIMPLE MICROSCOPES. 



the square bar makes with its summit, and by sliding that bar 

 through the socket, almost any position and elevation may be given 

 to the lens, that can be required for the purposes to which it may 

 be most usefully applied ; care being taken in all instances that 

 the ring which carries the lens should (by means of its joint) be 

 placed horizontally. At A is seen the position which adapts it 

 best for picking out' minute shells, or for other similar manipula- 

 tions ; the sand or dredgings to be examined being spread upon a 

 piece of black paper, and raised upon a book, a box, or some other 

 support, to such a height that when the lens is adjusted thereto, 

 the eye may be applied to it continuously without unnecessary 

 fatigue. It will be found advantageous that the foot of the micro- 

 scope should not stand upon the paper over which the objects are 

 spread, as it is desirable to shake this from time to time in order 

 to bring a fresh portion of the matters to be examined into view ; 

 and generally speaking, it will be found convenient to place it on 

 the opposite side of the object, rather than on the same side with 

 the observer. At B is shown the position in which it may be most 

 conveniently set for the dissection of objects contained in a plate 

 or trough, the sides of which, being higher than the lens, would 

 prevent the use of any magnifier mounted on a horizontal arm. — 

 The powers usually supplied with this instrument are one Lens of 

 an inch focus, and a second of either half or a quarter of an inch. 

 By unscrewing the pillar, the whole is made to pack into a small 

 flat case, the extreme portability of which is a great recommen- 

 dation. Although the uses of this little instrument are greatly 

 limited by its want of stage, mirror, &c. , yet, for the class of pur- 

 poses to which it is suited, it has advantages over perhaps every 

 other form that has been devised. 



35. Gairdners Doublet Microscope. — This little instrument, dis- 

 tinguished like the preceding for its simplicity and portability, is 

 adapted to quite a different class of purposes ; namely, the exami- 

 nation of minute transparent objects, especially those contained in 

 fluid, such as Animalcules, Desmidiacea? and Diatoinaceas, Urinary 

 deposits, &c. It consists (Fig. 27) of a Wollaston's Doublet (§ 18) 

 supported upon a handle, with which is also connected an elastic 

 slip of brass, carrying a ring that surrounds the projecting centre 

 of the under side of the doublet ; this ring is made to approach 

 nearer to, or to recede farther from, the doublet, by means of a 

 milled-headed screw which passes through the stem that supports 

 the latter, and bears upon the slip of brass that carries the former : 

 and to the side of it which is farthest from the doublet, a disk of 

 very thin glass is cemented. In using this little instrument, a 

 minute drop of the liquid to be examined is placed on the under 

 side of the thin-glass disk, — that is, on the side away from the 

 doublet,— and it is covered by another disk, which will be drawn 

 to the fixed disk and supported in its place by the capillary attrac- 

 tion of the fluid for both. The instrument is then to be so held, 

 that the eye, when applied to the Doublet, looks at the light 



