CONSTRUCTION OF THE MICROSCOPE. 51 



serves also as a watch-glass holder for preparing crystals by evaporation 

 over the spirit-lamp. A selection of materials required in preparing 

 and mounting objects is supplied in a rack of bottles sliding in the case; 

 and a set of instruments dissecting-needles, knife, forceps, dipping- 

 tubes, brushes, etc. with a supply of cover-glasses, cells, etc., are carried 

 in the three drawers; all the different contents of the case being readily 

 accessible when it is set open, as shown in the lower part of the figure. * 



COMPOUND MICROSCOPES. 



49. Of the various forms of Compound Microscope, the greater num- 

 ber may be grouped with tolerable definiteness into three principal 

 Classes : the First consisting of those high-class instruments in which the 

 greatest possible perfection and completeness are aimed at, without 

 regard to cost ; the Second including those which are adapted to all the 

 ordinary requirements of the observer, and wliicli can be fitted with the 

 most important Accessories ; 2 whilst to the Third belong the Students' 

 and Educational Microscopes, in which simplicity and cheapness are made 

 the primary considerations. Besides these, there is a class of Micro- 

 scopes devised for special purposes, but not suited for ordinary use. In 

 all, save the last, the same basis of support is adopted ; namely, a tripod 

 ' foot/ carrying a pair of uprights, between which the Microscope itself is 

 swung in such a manner, that the weight of its different parts may be as 

 nearly as possible balanced above and below the centres of suspension in 

 all the ordinary positions of the instrument. This double support was 

 first introduced by Mr. George Jackson, who substituted two pillars (;i 

 form which Messrs. Beck retain in their Large Microscope, Plate vn.,. 

 and is now adopted by Messrs. Ross, Plate v.) for the single pillar, con- 

 nected with the Microscope itself by a ' cradle joint/ which was previously 

 in use, and which is still employed in many Continental models (Fig. 45). 

 But in place of pillars screwed into the tripod base, the uprights are now 

 usually cast in one piece with the base, both for greater solidity and for 

 facility of construction (Fig. 39); while in most of the more recent 

 models an open framework is adopted (more or less resembling that first 

 devised by Mr. Swift, Fig. 50), which combines great steadiness with 

 lightness. Messrs. Powell and Lealand, it will be observed, adopt a tri- 

 pod support of a different kind (Fig. 48 and Plate VI.); still, however, 

 carrying out the same fundamental principle of swinging the Microscope 

 itself between two centres. An entirely new and very effective mode of 

 swinging the body has lately been introduced by Mr. George Wale of 

 New York (Fig. 44). Two different modes of giving support and motion 

 to the ' body ' will be found to prevail. In the first, which may be called 

 the Ross model (as having been originally adopted by Mr. Andrew Ross), 

 the ' body ? is attached at its base only to a transverse ' arm/ which, being 

 pivoted to the top of the ' stem/ is raised or lowered with it by the rack- 

 and-pinion action that works in the pillar to which the stage is fixed 

 (Fig. 52). The fundamental objection to this method is, that unless the 



1 The whole of this apparatus is supplied complete at the moderate cost of 1. 

 or, without the Compound body and inclined movement of the stand, at 2 10s. 



2 It is true that the most important of these accessories may be applied to some 

 of the smaller and lighter kind of Microscopes ; but when it is desired to render 

 the instrument complete by the addition of them, it is far preferable to adopt one of 

 those larger and more substantial models, which have been devised with express 

 reference to their most advantageous and most convenient employment. 



