160 MANAGEMENT OF THE MICROSCOPE. 



have been carefally cleansed, it should be sent to the maker (if 

 it be of English manufacture) to be taken to pieces, as the ama- 

 teur 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 unsci'ewed, cleansed, and 

 screwed together again. ISTot unfrequently an objective is ren- 

 dered 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 ex- 

 perienced in this kind of work ; since his own attempts to remedy 

 the defect are pretty sure to be attended with more injuiy than 

 benefit. 



79. Q-eneral Arrangement of the Microscope for Use. — The in- 

 clined position of the instrument, already so frequently referred 

 to, is that in which Observation by it can be so much more ad- 

 vantageously 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 Micro- 

 scope, upon the height of the observer's seat as compared with 

 that of the table on which the instrument rests, and lastly, upon 

 the tallness 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 selec- 

 tion of the object-glasses and eye-pieces to be employed, must be 

 entirely determined by the character of the object. Large ob- 

 jects presenting no minute structural features, should always he 

 examined in the first instance by the loivest powers, whereby a 

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

 of comparatively long focus and small angle of aperture, the pre- 

 cision of the focal adjustment is not of so much consequence as 

 it is with the higher powers, not only those parts can be seen 

 which are exactly in focus, but those also can be tolerably well 

 distinguished, which are not precisely in that plane, but are a 

 little nearer or more remote. When the general aspect of an 

 object has been sufficiently examined through low powers, its 

 details maybe scrutinized under a higher amplification ; and this 

 will be required in the first instance, if the object be so minute, 

 that little or nothing can be made out respecting it, save when a 

 very enlarged image is formed. The power needed in each par- 

 ticular case, can only be learned by experience ; that which is 

 most suitable for the several classes of objects hereafter to he 

 described, will be specified under each head. In the general ex- 

 amination of the larger class of objects, the range of power that is 

 aflEbrded by the " erector" in combination with the " draw-tuhe" 

 (§ 44), will be found very useful ; whilst for the ready exchange 



