56 construction of the microscope. 



Compound Microscopes. 



39. The various forms of Compound Microscope may be grouped 

 with tolerable definiteness into three principal Classes : the First 

 consisting of those instruments in which the greatest possible per- 

 fection 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 which can be fitted with the most 

 important of those Accessories, * whose use enables him not only to 

 work with more facility and certainty, but, in some instances, to 

 gain information with regard to the objects of his examination 

 which he could not obtain without them ; whilst to the Third belong 

 those in which simplicity and cheapness are made the primary con- 

 siderations. Besides these, there is a class of Microscopes devised 

 for Special purposes, but not suited for ordinary use. — In all, save 

 the last, the same basis of support is adopted, namely, a triangular 

 'foot,' from which arise two uprights; and between these 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 (a form which Messrs. Smith 

 and Beck still retain in their Large Compound Microscope, Plate vn.) 

 for the single pillar connected with the Microscope itself by a 

 ' cradle-joint ' (as in Fig. 37) which was previously in use ; but in 

 place of pillars screwed into the tripod base, a pair of flattened 

 uprights, cast in one piece with it, is now generally adopted, with 

 a view both to greater solidity and to facility of construction. 

 Messrs. Powell and Lealand, it will be observed, adopt a tripod 

 support of a different kind (Plates v. VI.) ; still, however, carry- 

 ing out the same fundamental principle of swinging the Microscope 

 itself between two centres ; and the same general arrangement is 

 adopted in the very ingenious form devised by Mr. Ladd (Fig. 36). 

 — Two different modes of giving support and motion to the ' Body * 

 will be found to prevail. One consists in its attachment at its base 

 to a transverse ' Arm,' which is borne on the summit of the movable 

 Stem, whose rack is acted on by the pinion of the milled-head, as in 

 Figs. 31, 33, 34, 39 ; whilst in the other, the body is supported 

 along a great part of its length by means of a solid ' Limb,' to which 

 is attached the pinion that acts on a rack fixed to the body itself, 

 as in Figs. 32, 35, and 38. The former, which may be described 

 as the Ross model, has the advantage of enabling the Body to be 

 turned aside by the rotation of the transverse Arm upon the summit 



* It is true that some of the most important of these Accessories may 

 be applied to 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. 



