85 



nical difficulties in the way of construction, for it must be made perfectly- 

 true and tight, and would have to be made of steel, because mercury would 

 act upon brass or gun-metal. Then if it required a large mercury chamber 

 this would add to the weight rather inconveniently. As regarded medical 

 students' microscopes, he thought they were already better than the use 

 that was made of them. 



Mr. Nelson did not think there was much mechanical difficulty in the 

 matter, as the apparatus could easily be made of pieces of iron tubing, and 

 could be drilled out so as to take steel rods fitting quite air-tight ; and he 

 thought that if the large rams could be made water-tight which carried 200 

 tons, this small one could be readily made mercury. tight under a pressure of 

 a few ounces. 



Mr. Parsons inquired how it was proposed to make the piston tight in this 

 case ; the pistons of hydraulic rams were packed by means of cup-leathers. 



Mr. Nelson thought it would not need any packing if it were drilled out 

 and fitted with perfect accuracy. 



Mr. Karop thought that accuracy would mean made so tight as to hold 

 the surfaces together as if they were soldered. 



Mr. Hailes did not think that this necessarily followed, because they had 

 almost the same thing in the pump of an air-gun, which was simply a steel 

 plunger, about 2in. X fin., fitted without any packing, and this was capable 

 of withstanding a very great pressure, so that he thought there would not 

 be much difficulty on that account. 



Mr. Nelson said that the pressure required need not be excessive ; in 

 fact, he should think lib. pressure would be quite sufficient, and that if the 

 rod was polished in, it would be sufficiently tight for the purpose. 



Dr. M. C. Cooke said that everyone who was in the habit of using the fine 

 adjustment was well aware of the jumps with which it worked, and of the 

 constant difficulty of getting at the exact distance required, and they would 

 no doubt all agree that something better was wanted for use with the 

 highest powers ; but with regard to the question whether this idea was 

 likely to answer best for all purposes, w r as one which could only be answered 

 experimentally — the proof of the pudding would be in the eating. 



A vote of thanks to Mr. Nelson for his communication was unanimously 

 carried. 



Mr. Karop exhibited a green caterpillar, found upon a geranium leaf, and 

 covered with parasitic larvae, presumably of some species of Ichneumon. 

 The specimen was handed round for the inspection of members, and some 

 further description of the larvae was given by the aid of drawings on the 

 black-board. The study of this large and interesting group of insects, of 

 which comparatively little was known, was strongly recommended. 



Announcements of meetings, &c, for the ensuing month were then made, 

 and the proceedings terminated with the usual conversazione. 



The following object was the only one exhibited : — 



Ptilote elegans, in fruit ... Mr. F. W. Andrew. ' 



Attendance — Members, 25; Visitor, 1, 



