MECHANICAL REQUIREMENTS. 89 



able quickly to set up and put away his instrument. "Where a 

 number of parts are to be screwed together before it can be 

 brought into use, interesting objects (as well as time) are not un- 

 frequently lost ; and the same cause will often occasion the in- 

 strument to be left exposed to the air and dust, to its great detri- 

 ment, because time is required to put it away ; so that a slight 

 advantage on the side of simplicity of arrangement, often causes 

 an inferior instrument to be preferred by the working microsco- 

 pist to a superior one. Yet there is, of course, a limit to this 

 simplification ; and no arrangement can be objected to on this 

 score, which gives advantages in the examination of difficult ob- 

 jects or the determination of doubtful questions, such as no 

 simpler means can afford. The meaning of this distinction will 

 become apparent, if it be applied to the cases of the "traversing 

 stage" (§ 37) and the "achromatic condenser" (§ 56). For al- 

 though the traversing stage may be considered a valuable aid in 

 observation, as facilitating the finding of a minute object, or the 

 examination (Jf the entire surface of a large one, yet it adds 

 nothing to the clearness of our view of either; and its place may 

 in great degree be supplied by the fingers of a good manipulator. 

 On the other hand, the use of the achromatic condenser not only 

 contributes very materially, but is absolutely indispensable, to 

 the formation of a perfect image, in the case of many objects of 

 a difficult class ; the want of it cannot be compensated by the 

 most dexterous use of the ordinary appliances ; and consequently, 

 although it may fairly be considered superfluous, as regards a 

 large proportion of the purposes to which the Microscope is di- 

 rected, whether for investigation or for display, yet as regards 

 the particular objects just alluded to, it must be considered as no 

 less necessary a part of the instrument than the achromatic ob- 

 jective itself. Where expense is not an object, the Microscope 

 should doubtless be fitted with loth these valuable accessories ; 

 where, on the other hand, the cost is so limited that only one 

 can be afforded, that one should be selected which will make the 

 instrument most useful for the purposes to which it is likely to 

 be applied. (See Introduction, pp. 60, 61.) 



In the account now to be given, of the principal forms of 

 Microscope readily procurable in this country, it will be the 

 Author's object, not so much to enumerate and describe the vari- 

 ous patterns which the several makers of the instrument have 

 produced, as, by selecting from among them those examples 

 which it seems to him most desirable to make known, and by 

 specifying the peculiar advantages wMch each of these presents, 

 to guide his readers in the choice of the Mnd of Microscope best 

 suited, on the one hand, to the class of investigations they may 

 be desirous of following out, and on the other, to their pe- 

 cuniary ability. He is anxious, however, that he should not be 

 supposed to mark any preference for the particular instruments 



