44 



procured from Mr. Swift by any one who still preferred to use ice and 



salt. 



The President said that the Secretary had suggested that he should say a 



few words as to the plan on which he had acted in presenting some slides to 



the cabinet that evening. In his Presidential Address he had suggested 



the desirability of having portions of a book on microscopy illustrated by a 



series of slides with references. He had endeavoured to carry out this idea 



to some extent by presenting to the Club 17 slides of subjects treated of by Dr. 



Carpenter, and he hoped that some other members would take up similar 



subjects and illustrate them for the benefit of the Club. He felt sure that 



such collections would prove of great help, especially to those who were 



beginning to work up any subject. 



Mr. J. W. Groves thought the suggestion was an extremely good one, and 

 hoped it would be the means of stimulating members to do something for 

 the Club. He was sorry to say that there were many who never seemed to 

 do anything, but he hoped that after what had passed every one would try 

 to assist. He did not think that it would be difficult to illustrate an entire 

 work if members would set about it in the right way. 



Mr. Deby exhibited and described an apparatus for obtaining monochro- 

 matic light for use with the microscope ; the beam of light from the 

 lamp being condensed by a large bull's-eye, was passed through a slit and 

 refracted by means of a bisulphide of carbon prism. A simple contrivance 

 enabled any portion of the spectrum to be employed for purposes of illu- 

 mination. 



The President inquired if this method possessed any special advantages 

 over the solution of copper and ammonia which had been recommended ? 



Mr. Deby said the light obtained by means of the prism was certainly 

 purer. 



A member said he used a large hollow bull's-eye, containing bisulphide of 

 carbon, which he thought would be of further advantage, as giving a much 

 larger dispersion. 



Mr. Ingpen considered the light of the pure spectrum better than that 

 obtained by passing white light through any coloured mixture. In the case 

 of finely marked objects the resolving power of objectives was greater with 

 blue than with red light. A simple method of obtaining a brilliant mo- 

 nochromatic light was very desirable under many circumstances. It would 

 be valuable with objects mounted in substances of very high refractive 

 index, to which attention had recently been called. It was impossible 

 to make use of the entire aperture of large angled oil-immersion lenses 

 upon dry-mounted lined objects, and when mounted in balsam the mark- 

 ings became almost invisible. It was therefore very desirable to mount 

 them in media of much higher refractive index, and the object became 

 more and more visible in proportion to the difference between its own re- 

 fractive index and that of the medium in which it was placed. He should 

 like to direct the attention of some of their chemical members to this sub- 

 ject, with the view of finding out what clear substances of high refractive 

 index were available for this purpose, and in which not only diatoms but his- 



