ZOOLOGY AND BOTANY, MICHOSCOPY, ETC. 221 



made of a special shape, being composed of two separate pieces, A and 1> 

 (fig. 87), one within the other, instead of one single piece. The upper 

 piece A bears above, as usual, the universal screw-thread for securing the 

 objective to the revolver or Microscope-tube ; below it tapers, conewise, 

 for receiving or retaining in its lower part the other piece B. The 

 latter is formed of two cylindrical parts of unequal diameter fitting 

 exactly into the conical part of A ; it bears on its lower part a screw- 

 thread, on to which the objective system L is to be screwed. A very 

 weak spiral spring R operates between the lower face of the diaphragm I) 

 and the upper part of the shoulder of B. The pressure of the spring 

 keeps B and A in close contact by their conical shoulders. B projects 

 5 mm. (exclusive of the threaded part) below A. In order to facilitate 

 the screwing-on of L and prevent useless rotation of B, a vertical groove 

 is cut in either A orB, in which a pin secured to the other engages. If, 

 in the action of focussing, the tube is lowered too far the preparation 

 will bear only the weight of the objective and will, by the operation of 

 the spring, be relieved from further pressure. The stop system, which 

 is shown in fig. 88, secures that the tube shall not be lowered beyond a 

 certain safe limit, which should be selected with regard to the highest 

 power of the operator's series. This stop is merely a screw V applied to 

 the side of the rackwork, and its head comes into contact with the upper 

 part of the limb when the tube is at the assigned depth. To exactly 

 determine the position of V the micrometer-screw is fully racked down. 

 Then the strongest objective, previously provided with a sliding funnel 

 of 5 mm. range, is screwed on and the tube is then racked up until the 

 lower extremity F of the objective is exactly on a level with the upper 

 surface of the stage P' P'. The point where the rackwork emerges from 

 the limb is V. 



Modern Fine Adjustments.* — ^'. Forgan lays down the various 

 qualities essential to a good fine adjustment and discusses some thirty- 

 four different types. Some of these types are of historical interest only ; 

 others exemplify the different constructions adopted by the best-known 

 modern makers of all countries. He concludes by summarising the 

 types before the public as three : — (1) The Powell and Lealand ;: 

 (2) the Zeiss ; and (8) the Watson Edinburgh Student's Microscope. 



(2) Eye-pieces and Objectives. 



Eye-piece Lens Interval as arranged for Achromatism, f — J. 

 Hunter illustrates his remarks on this subject by reference to fig. 81), 

 which is the double-lens achromatic eye-piece of a telescope. The lenses 

 are plano-convex. The points A B are the optic centres ; C D the planes 

 of the flat sides ; X the posterior focal centre (cardinal points) of the 

 field and eye-lens respectively ; the anterior focal centres coincide, in 

 this kind of combination, with the optic centres. The author points 

 out that various writers of high mathematical repute have variously 

 estimated the separation interval as D 0, A C, or A B ; X does not 

 appear to have been selected by any writer. He himself prefers AO. 



* Proc. Scottish Micr. Soc., iii. (1902) pp. 137-57. 

 t Tom. cit., pp. 294-9 (1 fig.). 



