journil ApruTFH-™'] I^oijal MicToscopical Society. 185 



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 %dbration to produce a considerable 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 hap- 

 pened that whilst my own instrument — a portable Binocular Micro- 

 scope weighing less than seven 'pounds, which is my usual travelling 

 companion — is constructed on the Jackson model, Professor Wy ville 

 Thomson was provided with an instrument of about the same scale, 

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

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

 tion under circumstances peculiarly critical. The difference 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-inch objective on Professor AVyville Thomson's. 

 In fact the image "danced" very perceptibly in the latter, even 

 when the l|-inch objective was in use. 



Now I purposely abstain (for obvious reasons) from naming 

 the Makers of these two instruments. But I think it well to say 

 this much, in order to meet the possible objection, that the difier- 

 ence lay rather in the worhmansM]) of the two instruments than 

 in their plan of construction, — that the advantage, if any, lay on 

 the side of the Eoss model. And my own very decided conviction 

 is, that the adoption of the principles of the Jackson model would 

 be decidedly advantageous, alike for /rs^class Microscopes, in which 

 the steadiness of the image when the highest powers are being em- 

 ployed ought to be a primary consideration, — for those secowcZ-class 

 instruments, which are intended, at a less cost, to do as much of 

 the work of the first-class as they can be made to perform, poHa- 

 hility being here of essential importance, — and for those third-chs,^ 

 instruments in which everything has to be reduced to its simplest 

 form, so as to permit the greatest reduction in their cost. 



