THIKD-CLASS MICEOSCOPES. 57 



of the Stem, — a movement which is often convenient, both as leaving 

 the Stage clear for dissection, &c, and as enabling the Objectives to 

 be more readily exchanged ; but it is subject to the disadvantage 

 that unless the transverse arm and the body are constructed with 

 great solidity, the absence of support along the length of the latter 

 leaves it subject to vibration, which may become unpleasantly 

 apparent when high powers are used, giving a dancing motion to 

 the objects. "With a view of preventing this vibration, the top of 

 the ' body ' is sometimes connected with the back of the transverse 

 arm by a pair of oblique 'stays' (Plate v.). The second, which 

 may be designated the Lister model, is decidedly superior in steadi- 

 ness, a perfect freedom from tremor being obtained with less 

 solidity, and therefore with less cumbrousness ; the mode in which 

 the rack is applied, moreover, in the microscopes of Messrs. Smith 

 and Beck (most of which are constructed upon this plan) gives to it a 

 peculiar smoothness and easiness of working ; but the traversing 

 movement of the body is sacrificed. Although some attach con- 

 siderable importance to this movement, the Author's experience of 

 instruments constructed upon both plans leads him on the whole to 

 give a preference to the second. 



In describing the instruments which he has selected as typical 

 of the Classes above enumerated, the Author wishes not to be under- 

 stood as giving any special preference to these, above what may be 

 the equally good instruments of other makers. The number of 

 Opticians who now construct really excellent Microscopes has of late 

 years increased greatly ; but their models are for the most part 

 copied more or less closely from those previously adopted for their 

 First- Class Microscopes by the three principal firms which long had 

 exclusive possession of the field. "Where any individual Maker has 

 introduced a real novelty, either in plan of construction, or in 

 simplification leading to reduction of price, the Author has thought 

 this worthy of special notice ; whilst the limits within which he is 

 restricted oblige him to content himself with a bare mention of 

 other Makers whose productions are favourably known to him. It 

 will be found most advantageous to commence with the Third Class 

 Microscopes, as the most simple in construction ; and to rise from 

 these, through the Second, to the First Class, — reserving the 

 Special Class for the conclusion. 



Third- Class Microscopes. 



40. Microscopes in which simplicity and cheapness are the primary 

 considerations, are rather suited for Educational purposes than for 

 Scientific observation. Yet it is unquestionable that very important 

 contributions to our knowledge 01 nature have been made by the 

 assistance of instruments not surpassing the least perfect of those 

 now to be described. And there is this advantage in commencing 

 Microscope-work with a Third-class instrument, that the risk of 

 injury to a more costly Microscope, which necessarily arises from 



