92 



CONSTRUCTION OF THE MICROSCOPE. 



FiQ. 15. 



sei-ver. At b is shown the position in which it may be most con- 

 veniently 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 h9rizontal 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 re- 

 commendation. Although the uses of this little instrument are 

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

 of purposes to which it is suited, it has advantages over perhaps 

 every other form that has been devised. 



28. G-airdner's Simple 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 

 examination of minute transparent objects, especially those con- 

 tained in fluid, such as Animalcules, Desmidiese and Diatomacese, 

 Urinary deposits, &c. It consists (Fig. 15) of a Wollaston's 

 doublet (§ 18), supported upon a handle, with which is also con- 

 nected an elastic slip of brass, carrying a 

 ring which surrounds the projecting centre 

 of the under side of the doublet; this ring 

 is made to approach nearer to, or to recede 

 further 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 furthest from 

 the doublet, a disk of very thin glass is 

 cemented. In using this little instrument, 

 a minute drop of the liqufd to be examined 

 is to be placed on the under side of the thin 

 glass disk, — that is, on the side away from 

 the doublet, — and it is to be covered by 

 another disk, which will be drawn to the 

 fixed disk, and supported in its place by the 

 capillary attraction 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 through the film of liquid ; and 

 when the focal adjustment is made by means 

 Gairdner-s Simple Microscope, ^f^jjg milledhcad, any particlcs this may 



contain, of a size to be brought into view by the magnifying 

 power employed, will be distinctly discerned. The instrument 

 is usually constructed with but a single power, adapted to the 

 class of objects for which it is to be employed ; thus for the pur- 

 poses of the botanical or zoological collector, a power of from 70 

 to 100 diameters is sufficient ; whilst for the examination of uri- 

 nary deposits, a power of 200 or more is desirable. It would not 



