MECHANICAL REQUIREMENTS. 45 



often causes an inferior instrument to be preferred by the •working 

 Microscopist 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 objects, 

 or in 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 Mechanical Stage 

 and the Achromatic Condenser. For although the Mechanical 

 Stage may be considered a valuable aid in observation, as facili- 

 tating the finding of a minute object, or the examination of 

 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 directed, whether for inves- 

 tigation 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 Objective itself. Where expense is 

 not an object, the Microscope should doubtless be fitted with both 

 these valuable accessories ; where, on the other hand, the cost is so 

 limited that only one can be afforded, that one sho\ald be selected 

 which will make the instrument most useful for the purposes to 

 which it is likely to be applied. 



In the account now to be given of the principal forms of Micro- 

 scope readily procurable in this country, it will be the Author's 

 object, not so much to enumerate and describe the various 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 knowu, and by specifying the peculiar 

 advantages which each of these presents, to guide his readers in 

 the choice of the hind 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 pecuniary ability. He is anxious, 

 however, that he should not be supposed to mark any preference 

 for the particular instruments he has selected, over those con- 

 structed upon the same general plan by other Makers. To have 

 enumerated them all, would obviously be quite incompatible with 

 the plan of his Treatise ; but he has considered it fair (save in one 

 or two special cases) to give the preference to those Makers who 

 have worked out their own plans of construction, and have thus 

 furnished (to say the least) the general designs which have been 

 adopted with more or less of modification by others. 



