90 CONSTRUCTION OF THE MICROSCOPE. 



he has selected, over those constructed upon the same general 

 plan by other makers ; to have enumerated them all, would ob- 

 viously 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 construction, and have thus furnished (to say the least) 

 the general designs, which have been adopted with more or less 

 of modification by others. 



Simple Microscope. 



26. Under this head, the common hand-magnifier or pocket- 

 lens first claims our attention ; being in reality a Simple Micro- 

 scope, although not commonly accounted as such. Although 

 this little instrument is in every one's hands, and is indispensable 

 to the Naturalist, as affording him the means of at once making 

 such preliminary examinations as often afford him most important 



fuidance, yet there are comparatively few who know how to 

 andle it to the best advantage. The chief difficulty lies in the 

 steady fixation of it at the requisite distance from the object, es- 

 pecially when the lens employed is of such short focus, that the 

 slightest 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 ob- 

 server, 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 of 

 such in the same handle, with an intervening perforated plate 

 of tortoise-shell (which serves as a diaphragm when they are 

 used together), will be found very useful. When such a mag- 

 nifying power is desired, as would require a lens of a quarter of 

 an inch focus, it is best obtained by the substitution of a " Cod- 

 dington" ( 19) for the ordinary double-convex lens. The handle 

 of the magnifier may be pierced with a hole at the end most dis- 

 tant 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 position, at any height at which it may be 

 convenient 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 otherwise 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 ( 28). 



27. Ross's Simple Microscope. This instrument holds an in- 

 termediate place between the hand-magnifier and the complete 

 Microscope ; being, in fact, nothing less than a lens supported 

 in such a manner, as to be capable of being readily fixed in a 



