144 MANAGEMENT OF THE MICROSCOPE. 



the end by the too frequent repetition of the process. The best 

 material for wiping glass is a piece of soft wash-leather, from 

 which the dust it generally contains has been well beaten out. — If 

 the Object-glasses be carefully handled, and kept in their boxes 

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

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

 hands of an inexperienced Microscopist, arises from the neglect of 

 precaution in using them with fluids ; 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 exhala- 

 tion 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 have been 

 carefully cleansed, it should be sent to the maker (if it be of 

 English manufacture) to be taken to pieces, as the amateur will 

 seldom succeed in doing this without injury to the work ; the 

 foreign combinations, however, being usually put together in a 

 simpler manner, may be readily unscrewed, cleansed, and screwed 

 together again. Not unfrequently an objective is rendered dim by 

 the cracking of the cement by which the lenses are united, or by 

 the insinuation of moisture between them ; this last defect occa- 

 sionally arises from a fault in the quality of the glass, which is 

 technically said to 'sweat.' In neither of these cases has the 

 Microscopist any resource, save in an Optician experienced in this 

 kind of work ; since his own attempts to remedy the defect are 

 pretty sure to be attended with more injury than benefit. 



109. General Arrangement of the Microscope for Use. — The 

 inclined position of the instrument, already so frequently referred 

 to, is that in which observation by it may be so much more advan- 

 tageously carried on than it can be in any other, that this should 

 always be had recourse to unless particular circumstances render 

 it unsuitable. The precise inclination that may prove to be most 

 convenient, will depend upon the ' build ' of the Microscope, upon 

 the height of the Observer's seat as compared with that of the table 

 on which the instrument rests, and lastly, upon the sitting height 

 of the individual ; and it must be determined in each case by his 

 own experience of what suits him best, — that which he finds most 

 comfortable being that in which he will be able not only to work 

 the longest, but to see most distinctly. — The selection of the 

 Object-glasses and Eye-pieces to be employed must be entirely 



