362 THE CANADIAN NATURALIST. [Sept. 



Microscopists. In order that the bearing of that experience 

 may be rightly understood, it will be desirable in the first instance 

 to examine the conditions on which tremor of the Microscopic 

 image depends. 



When the building in which the Microscopist is at work is 

 thrown into vibration as a whole, as by the passage of a heavily- 

 laden cart in the street outside, — or the floor of the room in 

 which he is seated is made to vibrate by the tread of a person 

 crossing it, — the Microscope and the observer move together; and 

 if the frame of the Microscope were perfectly rigid, there would 

 be no tremor of the image. For this tremor is the result, not of 

 the vibration of the Microscope as a whole, but either (1) of the 

 difference between the vibration of the Body as a whole and that 

 of the object on the Stage ; or (2) of the difference between the 

 vibration of the two extremities of the Bodv, the ocular and the 

 objective. 



Now it scarcely seems to me possible to conceive a method of 

 construction which should be more favourable to this differential 

 vibration, especially at the ocular end of the Body, than that 

 which is adopted in the Koss model. The long tubular body, 

 fixed only at its base, is peculiarly subject to it ; and although 

 the oblique stays with which it is sometimes furnished diminish 

 the vibrations of the tube, they by no means prevent it. The 

 transverse arm and the stem which bears it, each have a vibration 

 of their own ; and it is obvious that the nearer to the fixed point 

 of the whole system — which, in this arrangement, is the part of 

 the racked Stem embraced by the tube that carries the Stage — 

 flexure takes place, the greater will be the vibration of the Eye- 

 piece, which is at the greatest distance from that fixed point. 

 The only mode in which this vibration can be kept in check, is 

 the giving great solidity to the Stem, the Arm, and the Body, 

 especially the two former ; and this, while objectionable on 

 account of the cumbrousness which it imparts to the Micros- 

 cope-stand, is by no means effectual for its purpose; as every 

 Microscopist knows to his cost, when using very high powers 

 under any condition but that of the most perfect stillness of the 

 support. 



On the other hand, in the Jackson model, the support of the 

 Body along a great part of its length reduces to a minimum the 

 vibration of the tube, and the consequent differential vibration of 

 the eye-piece ; and even in those modifications of it in which the 



