1870.] MISCELLANEOUS. 363 



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 Ross model ; and as there is no fixed point 

 from which 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, microscopi3 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 sufficient to produce considerable pitching 

 and rolling, the motion, being imparted equally to the Microscope 

 as a whole and to the Observer, did not produce any tremor of 

 the image ; and the only difficulty lay in the maintenance of the 

 observer's own position, which was most effectually secured by 

 firmly grasping the leg of the table (which was fixed to the floor 

 of the cabin) between his knees. When the ship was going 

 under " easy steam," with either a fair wind or a light contrary 

 breeze, there was enough general vibration to produce a con- 

 siderable differential vibration in any Microscope liable to it, and 

 thus to occasion a decided tremor in the image even when only 

 moderate powers were employed. But when we were steaming 

 with full power against a head-sea, the general vibration became 

 so great as to be the severest test of the mechanical arrangements 

 of our Microscopes. Now, it happened that whilst my own 

 instrument — a portable Binocular Microscope weighing less than 

 seven pounds, which is my usual travelling companion — is con- 

 structed on the Jackson model. Professor Wyville Thomson was 

 provided with an instrument of about the same scale, but 

 heavier by some pounds, made upon the Boss model; and we 

 thus had an opportunity of fairly testing the two plans of con- 

 struction under circumstances peculiarly critical. The differ- 

 ence in their performance was even more remarkable than I had 

 anticipated. I found that I could use a l-4th-inch objective on 

 my own Microscope, with an even greater freedom from tremor 

 in the image than I could use a 2-3rds-iuch objective on Pro- 

 fessor Wyville Thomson's. In fact the image '^ danced " very 



