f>68 SUMMARY OF CURRENT RESEARCHES RELATING TO 



against the support and holds the roller in sure contact with the spiral. 

 The pin is so situated on the support 1c behind the contact-point of 

 roller and spiral (see fig. 154) that the spring-pressure and the strain 

 arising from the weight of the tube and connecting piece equalise them- 

 selves, so that within the groove of the swallow-tail there is no side 

 pressure on the sliding parts to affect the fine adjustment, and thus an 

 unequal wear and tear of the guide surfaces is avoided. One effect 

 resulting from the connection of the two spirals to the cam is that, 

 owing to the endless action, there can be no over-winding and therefore 

 no straining of the fine adjustment. Another advantage is that destruc- 

 tion, in the event of contact, of the cover-glass cannot occur, even if 

 rotation of the screw is continued, for in this case the connection 

 between roller and spiral is interrupted : the spiral then runs free, the 

 tube gently sets itself on the cover-glass which, in the designer's experi- 

 ence, is capable of sustaining the weight of the light aluminium tube 

 and the pressure exerted by the spring on the tube-holder and tube. 



In connection with the foregoing stand and with special reference to 

 the fine adjustment Mr. Nelson writes as follows : — 



" The circular issued by Messrs. Leitz of Wetzlar throws an interest- 

 ing side light on the ideas prevalent in Germany with regard to 

 Microscope construction. 



One of the causes assigned by Messrs. Leitz for the difference in con- 

 struction of the Continental and English models is very curious — it is as 

 follows : ' The shape of the Continental stand is largely determined by 

 the straight tube and the straight pillar, which are indispensable, owing 

 to the long prismatic guides in the pillar.' Many English Microscopes, 

 however, have straight tubes and longer prismatic guides than exist in 

 any Continental model, so that these points can hardly be said to deter- 

 mine the form of the model. 



The truth is, that the non-inclining Continental model, with its 

 small stage, was a cheap form, which did well enough to hold the 

 magnifying glasses for which it was originally designed, but the moment 

 it was used for purposes of delicate research it utterly broke down, for 

 it was found wanting in every important point. 



Messrs. Leitz admit that it fails when the stage is enlarged, and the 

 distance of the body from the limb is increased, and anyone can under- 

 stand how the weight of the body, acting at the end of the arm 

 (virtually a lever), must jam the slides. This surely is an important 

 point, for if the fine adjustment breaks down what is the use of the 

 instrument ? 



The method which Messrs. Leitz have adopted in their laudable 

 attempt to improve the radically bad Continental model is both complex 

 and quite inefficient. The body is raised and lowered by a cam, which 

 is rotated by an endless screw ; the speed attained is ^i* in- for each 

 revolution of the pinion. 



Passing over, without criticism, the complexity of this mechanism, 

 it can be seen at once where the appliance fails, for it is impossible to 

 determine the direction of the focussing movement, whether it is up- 

 wards or downwards. This, however, is a point of primary importance 



