480 SUMMARY OF CURRENT RESEARCHES RELATING TO 



cam, the motion is of the nature of an endless rotation, producing a 

 reciprocating motion of translation as alternately one or the other spiral 

 comes into play. The movement encounters no check in either direc- 

 tion, and consequently there is no possibility of injuring the mechanism 

 by rotation beyond any fixed limit. 



The movement possesses the further advantage that it effectively 

 guards against the destruction of a cover-glass and an object, since any 

 excessive depression of the tube would merely cause the objective to 

 gradually rest upon the cover-glass, whilst the cam detaches itself from 

 the roller without, however, exerting any mechanical pressure upon the 

 cover-glass, which with rare exceptions will readily bear the weight of 

 the tube with its optical and mechanical appendages. 



Immediately after its first description the micrometer movement was 

 criticised,* and the criticism has been reiterated on p. 66 of the present 

 volume of ' Zeitschrift f iii* wissenschaftliche Mikroskopie,' the objection 

 having been raised " that an observer working with this movement was 

 never certain whether the tube was rising or falling, and that uncertainty 

 of this nature was likely to become a serious impediment in ordinary 

 ocular observation no less than in photo-micrography." 



If this objection could be sustained, surely Leitz would ere this have 

 removed the supposed defect. It is not difficult to recognise the reason 

 why the maker and the thousand and one workers who already use 

 this mechanism fail to admit the validity of this criticism. As a matter 

 of fact, every practised microscopist focuses an object with a fair degree 

 of accuracy by the rack-and-pinion only, and passes on to the tine- 

 adjustment when he has obtained a pretty distinct image of his object. 

 He then turns the micrometer screw in one or the other direction with- 

 out experiencing any curiosity as to whether he is raising or lowering 

 the tube, being quite content to obtain the sharpest possible adjustment 

 in a minimum of time. The process is quite analogous to that occurring 

 in other optical instruments requiring a fine adjustment, i.e. in tele- 

 scopes, photographic lenses, optical lanterns, etc., where the observer is 

 solely concerned in obtaining a sharp image, whereas the direction in 

 which the mechanism moves does not interest him in the least. 



In very special cases, e.g. when examining the substance of a thick 

 preparation, an observer may consider it desirable that rotation of the 

 coarse- and fine-adjustment in one direction should caiTy the tube in the 

 same direction. In such a case he may with the aid of the two fixed 

 marks in the tube-carrier and a movable index on the shder easily ascer- 

 tain the direction in which the tube is moving. By giving the screw 

 spindle a few turns in the same sense the direction of motion may be 

 recognised at once. If its motion is not in agreement with that imparted 

 by the rack-and-pinion the rotation of the micrometer head should be 

 continued until the two movements become equidirectional. If, more- 

 over, the movable index be placed midway between the fixed marks, it 

 will be a long time before a change of direction occurs, since 80 com- 

 plete turns of the drum are available before the motion reverses. 



The introduction of the new micrometer screw and its transposition 

 to the front of the column have brought with them several improvements 



* See this Journal, 1903, p. 665. 



