46 CONSTRUCTION OF THE MICROSCOPE. 



Simple Microscopes. 



33. 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, — 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. The chief difficulty lies in the 

 steady fixation of it at the requisite distance from the object ; espe- 

 cially 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 tbey 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 tortoise-shell (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 sub- 

 stitution of a 'Coddington' (§ 19) 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 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 under- 

 stood 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 (§ 35). 



34. Boss's Simile Microscope. — This instrument holds an inter- 

 mediate place between the Hand-Magnifier and the complete Micro- 

 scope ; being, in fact, nothing else 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 foot, wherein is screwed a short tubular 

 pillar (Fig. 26), which is ' sprung ' at its upper end, so as to grasp 

 a second tube, also 'sprung,' by the drawing-out of which the 

 pillar may be elongated to about 3 inches. This carries at its upper 

 end a jointed socket, through which a square bar about 3^ inches 

 long slides rather stiffly ; and one end of this bar carries another 



