CLEANSING OF THE LENSES, ETC. 159 



powers, has a great tendency to injure the sight. Every Micro- 

 scopist who thus occupies himself, therefore, will do well, as he 

 values his eyes, not merely to adopt the various precautionary 

 measures already specified, but rigorously to obsei-ve the simple 

 rule of not continuing to observe, any longer than he can do so without 

 fatigue. 



78. Care of the Microscope. — Before the Microscope is brought 

 into use, the cleanliness and dryness of its glasses ought to be 

 ascertained. If dust or moisture should have settled on the 

 Mirror, this can be readily wiped off. If any spots should show 

 themselves on the field of view, when it is illuminated by the 

 mirror, these are probably due to particles adherent to one of the 

 lenses of the Eye-piece ; and this may be determined by turning 

 the eye-piece round, which will cause the spots also to rotate, if 

 their source lies in it. It may very probably be sufficient to 

 wipe the upper surface of the eye-glass (by removing its cap), and 

 the lower surface of the field-glass ;• but if, after this has been 

 done, tbe spots should still present themselves, it will be neces- 

 sary to unscrew the lenses from their sockets, and to wipe their 

 inner surfaces ; taking care to sci'ew them firmly into their places 

 again, and not to confuse the lenses of different eye-pieces. 

 Sometimes the eye-glass is obscured by dust of extreme fineness, 

 which may be carried off by a smart puff of breath; the vapor 

 which then remains upon the surface being readily dissipated, by 

 rapidly moving the glass backwards and forwards a few times 

 through the air. And it is always desirable to try this plan in 

 the first instance ; since, however soft the substance with which 

 the glasses are wiped, their polish is impaired in the end by the 

 too frequent pei'formance of the process. The best material for 

 wiping glass, is a piece of soft wash-leathei', from which the dust 

 it generally contains has been well beaten out. If the Object- 

 glasses be carefully handled, and be kept in their boxes when 

 not in use, they will not be likely to require cleansing. One of 

 their chief dangers, however, to which they are liable in the 

 hands of an inexperienced Microscopist, arises fi'om the neglect 

 of precaution in using them with fiuids ; which, when allowed to 

 come in contact with the surface of the outer glass, should be 

 wiped off as soon as possible. In screwing and unscrewing them, 

 great care should be taken to keep the glasses at a distance from 

 the surface of the hands ; since they are liable not only to be 

 soiled by actual contact, but to be dimmed by the vaporous ex- 

 halation from skin which they do not touch. This dimness will 

 be best dissipated, by moving the glass quickly through the air. 

 It will sometimes be found, on holding an object-glass to the 

 light, that particles either of ordinary dust, or more often of the 

 black coating of the interior of the microscope, have settled upon 

 the surface of its back lens ; these are best removed by a clean 

 and dry camel-hair pencil. If any cloudiness or dust should still 

 present itself in an object-glass, after its front and back surfaces 



