CONSTRUCTION OF THE MICROSCOPE. 4:3 



Makers of the instrument have produced; as, by selecting from among 

 them those examples which it seems to him most desirable to make known, 

 and by specifying the peculiar advantages which each of these presents, 

 to guide his readers in the choice of the kind of microscope best suited 

 on the one hand, to the class of investigations they may be desirous of 

 following out, and, on the other, to their pecuniary ability. He is 

 anxious, however, that he should not be supposed to mark any preference 

 for the particular instruments he has selected, over those constructed upon 

 the same general plan by other Makers. To have enumerated them all, 

 would obviously be quite incompatible with the plan of his Treatise; but 

 he has considered it fair (save in one or two special cases) to give the 

 preference to those Makers who have worked out their own plans of con- 

 struction, and have thus furnished (to say the least) the general designs 

 which have been adopted with more or less of modification by others. 



SIMPLE MICROSCOPES. 



42. Under this head, the common Hand- Magnifier or pocket lens first 

 -claims our attention; being in reality a Simple Microscope, although not 

 commonly accounted as such. Although this little instrument is in every 

 one's hands, and is indispensable to the Naturalist furnishing him with 

 the means of at once making such preliminary examinations as often 

 afford him most important guidance yet there are comparatively few 

 who know how to handle it to the best advantage. Tho chief difficulty 

 lies in the steady fixation of it at the requisite distance from the object; 

 especially when the lens employed is of such short focus, that the slight- 

 est want of exactness in this adjustment produces evident indistinctness 

 of the image. By carefully resting the hand which carries the glass, 

 however, against that which carries the object, so that both, whenever 

 they move, shall move together, the observer, after a little practice, will be 

 able to employ even high powers with comparative facility. The lenses 

 most generally serviceable for Hand-magnifiers range in focal length 

 from two inches to half an inch; and a combination of two or three such 

 in the same handle, with an intervening perforated plate of tortoiseshell 

 (which serves as a diaphragm when they are used together), will be found 

 very useful. When such a magnifying power is desired, as would require 

 a lens of a quarter of an inch focus, it is best obtained by the substitution 

 of a ' Coddington' ( 24), or, still better, of the Browning or the Stein- 

 heil Doublet ( 25), for the ordinary double-convex lens. The handle 

 of the magnifier may be pierced with a hole at the end, most distant 

 from the joint by which the lenses are attached to it; and through this 

 may be passed a wire, which, being fitted vertically into -a stand or 

 foot, serves for the support of the magnifying lenses in a horizontal posi- 

 tion, at any height at which it may be covenient to fix them. Such a 

 little apparatus is a rudimentary form (so to speak) of what is commonly 

 understood as a Simple Microscope; the term being usually applied to 

 those instruments in which the magnifying powers are supported other- 

 wise than in the hand, or in which, if the whole apparatus be supported 

 by the hand, the lenses have a fixed bearing upon the object. 



43. Ross's Simple Microscope. This instrument holds an intermedi- 

 ate place between the Hand-magnifier and the complete microscope ; be- 

 ing, in fact, nothing more than a lens supported in such a manner as 

 to be capable of being readily fixed in a variety of positions suitable for 

 dissecting and for other manipulations. It consists of a circular brass 



