184 Transactions of the \^C^l K Wo' 



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 Body, 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 Boss model. The long tubular body, fixed only 

 at its base, is peculiarly subject to it ; and although the obhque 

 stays with which it is sometimes furnished diminish the vibration 

 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 — the 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 vibra- 

 tion can be kept in check, is the giving great sohdity 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 Microscope-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 

 tube has but a short bearing, as the support is given to it in the 

 middle of its length, instead of at its lower extremity, the vibra- 

 tion equally affects its ocular and its objective extremities. The 

 form of the Limb makes the Body much less liable to vibration as 

 a whole, than when supported on the transverse Arm and vertical 

 Stem of the Boss model ; and as there is no fixed point from which 

 such vibration can commence, increasing in extent with the distance 

 from that point, the Body and Stage are much more likely to move 

 together, such motion imparting no tremor to the image. 



In the ' Porcupine ' Expedition for the Exploration of the Deep 

 Sea, in which I took part last summer, microscopic inquiry had to 

 be carried on under conditions very different from those which 

 obtain on shore. When our ship was lying- to under sail, even if 

 the swell was sufiicient to produce considerable pitching and rolhng, 

 the motion, being imparted equally to the Microscope as a whole 

 and to the Observer, did not produce any tremor of the image ; and 



