96 CONSTRUCTION OF THE MICROSCOPE. 



a hole in the stage, at one side, or in front of, its central perfora- 

 tion. The instrument is usually furnished with three magnifiers, 

 namely, an inch and half-inch ordinary lenses, and a quarter-inch 

 Coddington (§ 19) ; and these will be found to be the powers 

 most useful for the purposes to which this instrument is specially 

 adapted. The lenses, mirror, condenser, cross-bar, vertical stem, 

 and milled head, all fit into hollows cut for their reception on 

 the under side of the stage, and are then covered and kept in 

 place by the side flaps ; so that, when packed together, and the 

 flaps kept down by an elastic band, as shown in Fig. 17, b, the 

 instrument is extremely portable, furnishing (so to speak) a case 

 for itself. It may be easily made to serve as a Compound micro- 

 scope, by means of an additional stem and horizontal arm, car- 

 rying a light " body." The principal c^zsadvantages of this very 

 ingenious and otherwise most convenient arrangement, are that 

 it must be always used with the light in front of the observer, or 

 nearly so, since the side-flaps interfere with the access of side- 

 light to the mirror ; and that the obstruction of the side-flaps also 

 prevents the hands from having that ready access to the mirror, 

 which is convenient in making its adjustments. These incon- 

 veniences, however, are trifling, when compared with the great 

 facilities afforded for scientific investigation by the size and firm- 

 ness of the stage ; and the author can confidently recommend 

 the instrument for all such purposes, from much personal experi- 

 ence of its utility. 



Compound Microscope. 



The various forms of Compound Microscope may be grouped 

 with tolerable definiteness into two principal classes ; one con- 

 sisting of those instruments, whose size and general plan of con- 

 struction adapt them only for the ordinary methods of observa- 

 tion ; whilst the other includes those which are suited to carry 

 the various accessories, whose use enables the observer not only 

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

 to gain information respecting the object of his examination, 

 which he could not obtain without them. 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 

 patterns, which has been devised with express reference to their 

 most advantageous and most convenient employment. In nearly 

 all the instruments now to be described, the same basis of sup- 

 port 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 sus- 

 pension, in all the ordinary positions of the instrument. This 

 double support was first introduced by Mr. George Jackson, who 



