30 SELECTION AND t?SE 



obtained from a single lens, at least so far as clearness and 

 accuracy of definition is concerned. But when used as a work- 

 ing or dissecting microscope, they are open to the objection 

 that the distance at which they must be placed from the object 

 is very small, and hence it is frequently inconvenient to use 

 them for working upon objects. Thus, if we have a plano- 

 convex lens of a quarter of an inch focus, and one of three 

 quarters of an inch focus, and place them at a distance of a six- 

 teenth of an inch from each other, we will have a very good 

 magnifier which will enlarge objects about fifty times, but we 

 must place it at but a very short distance from the object. If 

 we separate the lenses a little, the definition will be improved, 

 but the working distance, as it is called, will be diminished. 

 Those who have studied optics are quite familiar with these 

 facts, but the ordinary reader does not always think of them, 

 and yet they are very important when we come to choose a mi- 

 croscope for working or dissecting purposes. 



Where two or more simple lenses are used together (without 

 being combined so as to form a compound microscope) the 

 power of the combination is always equal to the sum of the 

 powers of the separate lenses. Thus if we have a lens of half 

 an inch focus and one of one inch focus, one magnifying ten 

 and the other twenty diameters, the resulting power is thirty 

 and not two hundred times. In the compound microscope, on 

 the other hand, the combination of an objective magnifying 

 twenty diameters with an eye-piece magnifying ten diameters, 

 gives a magnifying power of two hundred diameters. 



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

 and may be obtained of almost any power within the useful 

 range of a singe lens. They are called "eye-glasses'' because 

 when in use they are held by the muscles around the orbif oi 

 the eye, and consequently require no extraneous support. 

 Fig. 4 shows the bell-mouthed form of the frame, which enables 

 us to secure* this result. The eye-glass ordinarily used by 

 watch-makers. magnifies about eight times, but glasses magnify- 

 ing 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 focus than the 



