Suggestions regarding the Mechanical Design of Microscopes. 135 



triangular section. In this channel engage three steel balls about 

 j% *ot' an inch in diameter. These balls are held in conical cups 

 attached to the centreing screws and the spring buffer. The stage 

 is supported by and rotates on this ball-bearing while the balls 

 communicate the centreing motion to the stage. The axes of the 

 centreing screws, buffer and clamp screw are all on one plane at 

 right angles to the optic axis. The pressure of the spring buffer 

 should be considerable, to prevent the stage being dislocated 

 from its three-point bearing. Eotation is by hand, and the move- 

 ment is locked by the action of a screw located near to the spring 

 buffer, the point of the former engaging in fine serrations cut in 

 the bottom of the channel when the locking screw is screwed 

 home. This provides a positive lock wliich does not tend to press 

 the stage upwards and so throw the object out of focus, as is 

 commonly the case. An index to record the angle of rotation of 

 the stage is mounted on the buffer pin, which is provided with a 

 rib to prevent its rotation. 



The buffer pin and clamp screw should be mounted in the front 

 edge of the stage, and the centreing screws at either side of the 

 limb. 



Improvememts in the Substage. 



Now that nearly every microacopist, including laboratory 

 workers since the introduction of the high-power dark ground 

 illuminator, requires to change his substage condenser frequently 

 and easily, and with the minimum of labour in re-centreing, it would 

 appear that the standard ring fitting for condensers is obsolete. The 

 changing of apparatus is laborious, and sometimes entails the in- 

 clining of an instrument for which the lighting has been arranged 

 when in a vertical position, and invariably entails re-centreing of 

 the apparatus to be used. 



I therefore suggest that the " ring " should be done away with 

 and replaced by a sliding changer of the same construction as the 

 Zeiss objective changer, but more massive, to allow of the use of 

 large-sized condensers. 



Each upper slide with its own condenser system would have 

 its own centreing gear, as in the Zeiss changer. 



These slides might be made with two standard threads, the first 

 to be R.M.S. objective size, taking most English condensers, and the 

 second the same thread as the Watson Holoscopic Condenser (dry), 

 which would also take the Zeiss, Abbe and many other large 

 patterns. 



This system would permit of all the condensers in use being set 

 to work centrally with all the objectives, if the latter were provided 

 with their own centreing changers. Where a revolving nose-piece 

 was used, it would usually suffice in medical laboratories where the 



