734 PROCEEDINGS OF THE SOCIETY. 



which he further illustrated by drawings on the board. Prof. 

 Macintosh had been giving some of his knowledge and skill to fishery 

 observations, which had been rendered possible by the facilities 

 afforded by an enlightened fishery board in Scotland. Some models 

 deposited at the Natural History Museum at South Kensington (after 

 Prof. Ludwig) were said to show these knobs admirably. 



Mr. F. R. Cheshire exhibited a device for the better examination 

 of bacteria in culture tubes. He said that it had been often com- 

 plained that whilst bacteria were growing in glass tubes, the cylindrical 

 form of the tube so distorted the appearance of the contents, that it 

 was almost impossible to make any observations upon them under the 

 Microscope. The difficulty had been felt by every one who wished 

 to examine anything inside a glass tube, and, to endeavour to obviato 

 it, the use of tubes with flat sides had been suggested. These to a 

 certain extent answered the purpose, but they were expensive, and 

 were also very troublesome in use, because the cotton plug could not 

 be got to fit properly. 



The contrivance he was about to mention was very simple, and 

 was intended to enable any one to use the ordinary round test-tubes 

 without being subject to the usual difficulties. The first plan adopted 

 was that of placing the tube in a trough of water and then looking 

 at it through the front of the trough. This was found to diminish 

 the aberration very much, but it did not get rid of it altogether, and 

 was therefore only available under very low powers. Water having 

 a refractive index of about 1 ■ 333, and alcohol of about 1 • 374, by 

 adding water to alcohol, a mixture having a refractive index of any- 

 thing between the two could be obtained according to the proportions 

 used. The gelatin culture material had a refractive index rather 

 higher than that of water, and the interposition of the glass added 

 something to this. The trough which he used for the purpose had 

 been made under his directions by Mr. Curties, and had a front of 

 rather thin glass, the bottom being sloped in such a way as to cause 

 a tube placed in the trough to lie always near to the front. The tube 

 to be examined was placed in the trough with some water, and then 

 alcohol was added until the proper density was arrived at, and by 

 this means it was quite possible to use a 1/2 in. objective effectively. 

 It was, of course, desirable to have some ready method of testing the 

 liquid, so as to ascertain when the best correction was reached. 

 Perfect correction was of course impossible, as the centre of the 

 cylinder would have a longer focus than the parts near the limb. The 

 plan he adopted was to put opposite a source of light an upright 

 rod, some two or three feet behind the tube, and then, when water was 

 poured in, the rod would appear to be magnified ; but by adding 

 alcohol a point was reached where there was no magnification pro- 

 duced ; this was the best point for use. A specimen of the trough 

 charged with liquid and containing a tube ready for examination was 

 handed round for inspection, and the construction of the base of the 

 trough was further illustrated by a drawing on the board. 



