PROCEEDINGS OF THE SOCIETY. 259 1 



The propelling power was obtained by a steel wire passing round a shaft, 

 but he did not know what was the strain upon this, as it appeared to 

 depend upon a spring which kept it taut. If so, he thought it could not 

 sustain any considerable weight of accessory apparatus — such, for ex- 

 ample, as polarising and analysing prisms. Another point was as to the 

 arrangement made for the focussing screw of the substagc. Mr. Lucas 

 said this was in a very convenient position, and, if regard was had to the 

 substage motion only, that was no doubt so ; but it seemed to him that 

 if they wanted to mount either a mechanical stage or a revolving stage 

 upon the fixed stage, the substage screw would be inconveniently in the 

 way. He should like to add his personal tribute to the ingenuity of the 

 design, and to say how much interest he had himself felt in the applica- 

 tion of this idea. He thought the supporting pillar looked rather weak, 

 for although the instrument was a very heavy one, the pillar was of a 

 somewhat narrow " scantling," and the rigidity of a Microscope stand 

 was not so much a question of strength against heavy strains, but of 

 strength against vibration, and, judging by the construction, he should 

 think this instrument was likely to prove a little sensitive to such dis- 

 turbances. 



The Chairman was sure that it would be felt by all that Mr. Lucas 

 was a very bold man to have brought a Microscope of his own construc- 

 tion to a meeting at which so many experts were present, and it spoke 

 well for his courage that he was not afraid to submit it for criticism. 

 He begged to thank the author for coming that evening to exhibit this 

 new instrument and to explain its merits so fully, and he felt sure that 

 the remarks made upon it Avould not be without value, either for the 

 writer of the paper or those who had heard it read. 



In reply to the questions which had been raised, Mr. Lucas pointed 

 out that the form of slide adopted in lathes was no guide to the best 

 form for the Microscope, the geometric principle being necessarily 

 sacrificed in the former owing to the need for large surfaces to meet the 

 heavy stresses involved. With regard to the friction between the wire 

 and barrel of the coarse adjustment, the tension on the latter was, in the 

 case of the instrument before them, between 5 lb. and G lb., whilst the 

 breaking strain of the wire was ~1'1 lb. It would not, therefore, be 

 possible to further increase the tension of the wire Avithout using one 

 which was thicker, and a thicker wire would pass its elastic limit owing 

 to the small size of the barrel. It was, however, amply sufficient to 

 support any weight which the tube might have to carry. The coarse- 

 adjustment tube was extremely light, so that with any ordinary apparatus 

 attached to it the weight would not be great. He had tested it by 

 setting it up upon a table at which he was working, and had focussed it 

 carefully with an immersion objective of 1 • 4 N. A, and after leaving it 

 for five hours under these conditions, he found that the focus had re- 

 mained perfectly unaltered. He might also point out, that as regarded 

 the position of the focussing screw of the substage, this was such that no 

 part of it which came down to the level of the stage projected further 

 forward than the front of the limb. The screw could not, therefore, 

 interfere with anything which might be placed on the stage. As to the 

 strength of the pillar, it was made of solid brass, \\ in. by % in., which, 

 he thought should be sufficient to prevent vibration. 



