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VJ. — A New Form of Fine Adjustment. 

 By E. B. Stringer, B.A. 



{Read 16th May, 1900.) 



In the accompanying fine adjustment I have endeavoured to overcome 

 those imperfections which proved troublesome in photomicrographic 

 and also in visual work. The main points of its construction will be 

 seen in fig. 103, and are as follows. 



The limb, which is of the Jackson type, is prolonged into a vertical 

 pillar P, of triangular section, placed close behind the coarse adjustment 

 and body, so that any shake, should it exist, is not magnified by an 

 intervening arm. The lever is of the second order, having its fulcrum 

 F at the bottom and just in front of the pillar, and bearing on the 

 moving part at a point exactly underneath it ; this point being so 

 near the fulcrum as to render the movement extremely slow. 



The moving part, which carries the coarse adjustment and body in 

 front of it, has a back plate attached to it by eight screws, with which, 

 if necessary, its fit may be adjusted. It has two prolongations which 

 pass downwards on each side of the limb, and receive a transverse 

 steel pin in their extremities. This pin carries a small roller R, of 

 hard steel, upon which the lever bears, downwards. 



The opposing spring is in a recess at the top of the pillar, and 

 draws the roller upwards against the lever. The action of the roller 

 is to abolish friction at this point, and to convert the movement of the 

 lever, which is of course the arc of a circle, into the straight move- 

 ment of the sliding part, without the least tendency to the production 

 of a shake in a backward or forward direction. 



It will be noticed that in virtue of the position of the spring the 

 weight of the body is supported by it ; so that, though this weight is, 

 as it must be, considerable, it is in practice reduced to a small minus 

 quantity ; and the pressure of the lever on the micrometer screw is 

 thus very slight, being in fact only just enough to take up its back- 

 lash. The finest screw may in consequence be used without danger 

 of its suffering from wear. 



Further it will be seen that the weight of the body bears most 

 upon the spring when the Microscope is in a vertical position, whilst 

 in the horizontal position it is quite inoperative ; so that for the latter 

 position tbe same spring is much too strong, and by exerting an undue 

 strain might cause lateral movement. It is in consequence of this 

 that much of the trouble occurs in photomicrography ; fine adjust- 

 ments which are perfectly satisfactory in the vertical or inclined 

 positions, sometimes working badly when set horizontally. 



