MANAGEMENT OF THE MICROSCOPE. 135 



-extreme fineness, which may be carried off by a smart puff of breath; the 

 yapor 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 repetition of the pro- 

 cess. 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 inexperi- 

 enced 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 lisDble not only to 

 be soiled by actual contact, but to be dimmed by the vaporous exhalation 

 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 camePs-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 occasionally 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. 



135. 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 advantageously carried-on 

 than in any other, that recourse should always be had to it, unless par- 

 ticular circumstances render it unsuitable. The precise inclination that 

 may prove to be most convenient will depend upon the f build ' of the 

 Microscrope, 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 com- 

 fortable being that in which he will be able not only to work the longest, 

 but to see most distinctly. The selection of the Objectives and Eye- 

 pieces to be employed must be entirely determined by the character of 

 the object. Large objects presenting no minute structural features should 

 always be examined in the first instance by the lowest powers, whereby a 

 general view of their nature is obtained; and since, with lenses of com- 

 paratively long focus and small angle of aperture, the precision of the 

 focal adjustment is not of so much consequence as it is with the higher 



