52 THE MICROSCOPE AND ITS REVELATIONS. 



transverse arm and the body are constructed with great solidity, the ab- 

 sence, of support along the length of the latter leaves its ocular end sub- 

 ject to vibration, which becomes unpleasantly apparent when high powers 

 are used, giving a dancing motion to the objects. With the 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' (Fig. 48); 

 but the usual plan is to obtain the requisite firmness by the thickness 

 and weight of the several parts. In the other, which may be termed the 

 Jackson model, and which was first adopted by Mr. James Smith (the 

 predecessor of Messrs. Beck), the body is supported along a great part of 

 its length on a solid ' limb ' whereby its i vibration ' is reduced to a 

 minimum ; and the rack, which is acted on by a pinion working in that 

 limb, is attached to the body itself ; a construction that gives a great 

 smoothness and easiness of working (Plate vn.). Having made use of 

 instruments constructed by the best makers on both models, the Author 

 has no hesitatation in expressing his preference for the second, which is 

 now employed by most English makers (having been adopted by Messrs. 

 Ross themselves in their more recent instruments), and by nearly all 

 American. He regards it as certain that greater freedom from vibration 

 can be obtained in lightly -framed Microscopes constructed on the Jack- 

 son model, than in any but the most solid and cumbrous of the old Ross 

 pattern ; and feels assured that the principle of supporting the ' body ' 

 along a great part of its length (which may be applied in a variety of 

 modes) will in time supersede that of fixing it by its base alone, which is 

 oviously the mode least adapted to prevent vibration at its ocular end. 



In describing the Instruments which he has selected as typical of the 

 several groups 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 those 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 high-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 favorably known to him. It will 

 be found most advantageous to commence with the Educational and 

 Students' Microscopes, as the most simple in construction ; and to pro- 

 ceed from these through the /Second to the First- Class Microscopes, 

 reserving to the last the group of instruments adapted for /Special pur- 

 poses. 



THIRD-CLASS MICROSCOPES. 



50. Very important contributions to our knowledge of Nature have 

 unquestionably been made by the assistance of instruments not surpassing 

 the least perfect of those now to be described. And there is this advan- 

 tage in commencing Microscope-work with a simple and low-priced 

 instrument that the risk of injury to a more costly Microscope, which 

 necessarily arises from want of experience in its use, is avoided; whilst 

 the inferior instrument will still be found serviceable for many purposes, 



