OF THE MICROSCOPE. 25 



are very important when we come to choose a microscope for 

 working or dissecting purposes. 



Watch-Makers' Eye-Glasses. These are well known, 

 and may be obtained of almost any power within the useful 

 range of a single lens. The eye-glass ordinarily used by watch- 

 makers magnifies about eight times, but glasses magnifying 

 twenty diameters are not uncommon. Glasses of the latter 

 power are usually doublets, that is, they consist of two lenses, 

 arranged together, one being of much longer f.ocus than the 

 other. If well-made they give excellent definition and a large 

 field, and, when mounted on a stand, are very serviceable as 

 dissecting microscopes*, especially in working upon coarse 

 objects, and picking out shells, the larger foraminifera, etc. 

 Their form enables us to support them by means of a small 

 wire ring, arranged as in a retort stand, and the large bell- 

 mouth of the frame prevents any light from entering the eye, 

 except that which has passed through the lens. They are very 

 cheap, and any intelligent boy can make a tolerable stand for 

 one. The same stand will answer for several glasses of differ- 

 ent powers. 



Engravers' Glasses. These are mounted in frames, in 

 the same manner as the watch-maker's eye-glass, but as they 

 are larger, and are therefore not so readily held in the eye, 

 after the fashion of the latter, they are always used with a stand 

 of some kind. Those of the best quality are, in general, 

 doublets, which give a large field of view, with very good 

 definition, and they are altogether the best microscopes for 

 examining banK bills, fine engravings and similar objects. 



*The term dissecting microscope is applied to all microscopes used for 

 working upon objects under moderate magnifying powers. They are used 

 not only for dissecting, properly so called, but for the study of botany, 

 mineralogy, etc., as well as for numerous investigations in the arts. A 

 good microscope of this kin.d is absolutely indispensable to those who 

 hope to do more than merely look at objects prepared by others. In sub- 

 sequent paragraphs we describe some of the best instruments of this 

 kind. A complete dissecting microscope should be furnished with stand, 

 mirror, etc., and if the student can afford it, there should be some good 

 mechanical means of adjusting the focus, 



