143 



He was then exhibiting AmpTwpIeura pellucida under the microscope with a 

 power of 2,300 diameters, and it was distinctly beaded in a very striking 

 manner. 



The President observed that it was a pleasure to the Club to have Mr. 

 Nelson come up to show his fine preparations, and invited remarks or 

 questions on the new lens. 



Mr. Ingpen said with regard to the brass and glass portion of Mr. Nelson's 

 observations it certainly was difficult to overrate the importance of the pro- 

 duction of such objectives as those he had mentioned. All fine and delicate 

 work would have to be verified by oil immersion objectives, which were the 

 greatest advance of the last few years, and rendered a great deal of the work 

 of examination and recognition of minute organisms easy, which a little 

 while ago was practically impossible. It was a good illustration of the 

 delicacy of such work, that it was found necessary to fit a fine adjustment to 

 the achromatic condenser. Many fine achromatic condensers were at present 

 almost useless owing to the imperfect rack adjustments of the substage, which 

 were hardly ever sufficiently well made. 



The President inquired if Mr. Nelson had used fluids of high refractive 

 index for immersion glasses, or was there any particular difficulty in doing 

 so ? He remembered that several years ago Sir David Brewster tried to make 

 lenses of very dense fluids. 



Mr. Nelson replied that he had no experience of anything besides ordinary 

 oil of cedar and oil of fennel. He had not tried oil of pimento. 



Mr. Ingpen remarked that with regard to that question the principles were 

 now pretty well understood. The fluid for a homogeneous lens was required 

 to be of the same refractive index and dispersive power as the crown 

 glass of which the front lens was made. With regard to the visibility of 

 objects in fluids of various refractive indices, that point was also governed by 

 well-known principles. In the case of a diatom the refractive index of 

 which would be about 1'4, if placed in air we got a contrast between the refrac- 

 tive index of air 1 and the diatom 1*4. If the diatom were placed in water, 

 with refractive index of 1*33, there was less contrast. In balsam or oil of 

 turpentine a delicate diatom became almost invisible. If the refractive index 

 of the mounting medium were much increased, as in the case of phosphorus 

 of refractive index 2*1, a very great contrast was obtained in the other 

 direction ; and the diatom would be very visible by this contrast, with the 

 additional advantage that the whole aperture of the immersion lens was 

 used, which was not the case with an object mounted in air. 



Dr. Ralph, of Victoria, South Australia, made some remarks upon the 

 action of hydrocyanic acid combined with ammonia on the tissues of certain 

 plants, for instance the vine. A thin slice of the plant when very tender 

 showed a remarkable coloration, varying from the edge inwards. In certain 

 vascular tissues the treatment gave a distinct red or claret colour, which 

 passed through the tubes from one end to the other, giving an artificial in- 

 jection. The Virginian Creeper showed a centre of a bright ruby colour. 

 After the lapse of a quarter of an hour all this colour disappeared. The 

 only explanation he could offer was the action of iron in the tissue. 

 Formic acid in its nascent form attacked the iron and gave the transient 



