PROCEEDINGS OF THE SOCIETY. 569 



it had been more lately (1884) described by Feussner; but the credit 

 to wbicb Bertrand was entitled was in the use of a cement which had 

 the high refractive index of 1-66. The great advantage which this 

 prism possessed over the Nicol prism, apart from its cost, was in the 

 larger field which it was possible to obtain. One defect in the prism 

 arose from the great density of the glass, which, being due to the 

 large percentage of lead which it contained, made it very apt to get 

 tarnished. He suggested, as an effectual preventive of this evil, that 

 a piece of thin microscopic glass should be cemented upon each end, 

 which, by keeping off the air, would entirely preserve the surface 

 without impairing the efficiency of the prism in any way. 



Mr. Madan also exhibited another form of polarizing prism, which 

 was a modification of Ahrens's double-image prism, made by including 

 a thin film of air.* 



The President said the Society was much indebted to Mr. Madan 

 for these details, which showed a great amount of ingenuity. 



Mr. Cheshire said it was of course of great value to be able to get 

 such a large field as in Mr. Madan's modification of Ahrens's prism, 

 but it occm-red to him that there must be a considerable loss of light 

 in the case of the ordinary ray in passing from the first prism into 

 the air space. 



Mr. Madan said no doubt there was a loss of light in this case : 

 but this was a disadvantage which the prism suffered in common with 

 Foucault's and other prisms. Something must necessarily be sacri- 

 ficed to obtain greater advantages in other directions. He thought 

 that as regarded light Bertrand's was certainly more effective. 



Mr. Dowdeswell exhibited some septic microbes from high alti- 

 tudes, and detailed experiments by M. Freudenreich as to bacterial 

 germs found at various heights, notably upon the Niesen, at an eleva- 

 tion of about 7500 ft. (Cf. supra, p. 511.) 



The President regarded every addition to their knowledge in this 

 direction as being of the utmost value, and remarked that everything 

 pointed to the necessity of a thorough working out of the correlation 

 of the whole of the specific forms. When that was done they would 

 have gone a long way nearer towards the solution of the problem of 

 their connection with epidemic disease. 



Dr. Maddox said he had been in correspondence with M. Freu- 

 denreich for some time, from whom he obtained the specimens of the 

 organism, and was glad to learn from Mr. Dowdeswell's careful ex- 

 amination that it was only a septic organism, and not a pathogenic 

 form. Great pains had been taken in carrying out these observations, 

 as would be seen from the fact that all the apparatus had to be carried 

 up high mountains. It was found that the tubes originally provided 

 were difficult to use, and M. Freudem^eich had substituted ground caps 

 for the narrow end. He (Dr. Maddox) had suggested to him a form of 

 bottle which he thought would fulfil all the required conditions and 

 be also portable. This bottle had a cap fitting hermetically over the 



* ^'ol. iv. (1884) p. 965. 



