816 SUMMAET OF CUKKENT RESEAECHES RELATING TO 



" Eoss model " is virtually at an end, in consequence of a discorery 

 ■vvMcli had not been made when the question originally arose. 



The advantage of the Jackson model was the increased steadiness 

 which was obtained by the support given to the body-tube throughout 

 the greater part of its length, a support which was wanting in the 

 Eoss model. 



The advantage of the Eoss model consisted in the greater efficacy 

 of the fine adjustment, which could be worked by a relatively long 

 lever, instead of being confined, as in the Jackson model, to a very 

 short one in front of the lower end of the body-tube. 



The invention of the method of fine adjustment by which the 

 whole of the body-tube is moved and not merely an inner tube, 

 carrying the objective at the end of the body, at once deprived the 

 Eoss model of the advantage which it previously possessed, and left 

 the Jackson model necessarily the preferable form.* 



The Eoss model is no longer made by the firm whose name it 

 bears, but it has in its day enjoyed so much celebrity, that in order to 

 record it for future reference we print here a figure of it (Fig. 177) 

 which has not previously appeared in the pages of this Journal or 

 its predecessors, other than in the advertisement covers, which are but 

 rarely bound up. 



Fine Adjustment by the Eye-piece. — Professor L. Eanvier re- 

 commends f that the eye-pieces of monocular instruments should be 

 capable of being moved up and down in the draw-tube for the purpose 

 of focussing with high powers. The difficulty of observing with such 

 powers and the fatigue which results are due in great part to the 

 difficulty of exactly focussing the object. The eye of the observer 

 attempts to complete what is wanting in the instrument, and to 

 accommodate itself as much as possible, and it is this fatiguing effort 

 of accommodation which is avoided by the movement of the eye-piece. 

 Apart from the fact that the movement of the eye-piece obviates the 

 necessity for great care in focussing, so as not to displace the image 

 or break the cover-glass, it is a great advantage that with the very 

 exact focus which can thus be obtained it is much easier to determine 

 the superposition of the planes in the case of very small objects — for 

 instance, whether a fibril! a close to a cell passes below or anastomoses 

 with it. As the eye-piece must be very appreciably displaced to very 

 slightly change the point of distinct focus, we " are able to resolve 

 very easily a series of hitherto disputed problems." 



The apparatus which Professor Eanvier uses is shown in Fig. 178. 

 It consists of two brass rings a and 6, united by the rackwork c, so 

 that the distance between the rings can be increased or diminished. 

 The ring a is fitted to the top of the tube of the Microscope, the eye- 

 piece is placed in h, and by turning the milled head d the eye-piece 



* Dr. Carpenter "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 obviously the 

 mode least adapted to prevent vibration at its ocular end." — ' The Microscope,' 

 &c., 6th ed., 1881. 



t ' Traite technique d'Histologie,' Part 1, p. 11. (Svo, Paris, 1875-8.) 



