885 



difficult to persuade a biologist that it did not matter how the 

 material was treated. 



Mr. Ingpen thought it would hardly be possible to count the 

 striae without some previous preparation, although it might, 

 perhaps, be done by mounting in some of the higher refractive 

 media, such as sulphide of arsenic. He had found that in the 

 case of some of the smaller specimens of Rhomhoides when they 

 had been kept for a long time, so as to let the protoplasmic 

 matter decay, and were then mounted in one of these media, the 

 result was fairly satisfactory. Mr. Meates used with success 

 bromide of antimony and bromide of arsenic, the index of which 

 was much higher than that of styrax. He had found this to be 

 good. It was very fugitive, but did not require much heat. It 

 was then much easier to count the lines, there being as much 

 difference as between lines made with a faint pencil and with 

 ink. 



Mr. Mainland said that he had mounted some diatoms about 

 nine years ago in Barff's boro-glyceride, and these had kept 

 perfectly good to the present time. He used a saturated 

 solution, and ringed the covers with Miller's cement. 



The President thought this would be a valuable addition to 

 their mounting media. It would also be of great use if they 

 could work successfully with quinidine, which gave them the 

 best results yet obtained. It was beautifully white, and the 

 refractive index was very high. The first slide he possessed 

 had no cement round it of any sort, and it was still perfect. A 

 friend showed him how to mount with quinidine, and it seemed 

 a very simple process, but when he tried it it generally crystal- 

 lized in about five minutes. As to counting striae, he thought 

 he could give them a wrinkle. It was a method much the same 

 as one employed to discover the number of lines to an inch in 

 test objects by Sir D. Brewster, -who said that "he saw dis- 

 tinctly the fringes of colour produced by interference, and on 

 measuring the angular distances of the first red fringe from the 

 lio-ht he found that the distance of the lines, or rather the 

 diameter of one black line and half the bright space between 

 the lines, varied from the 10,000th to the 22,000th of an inch." 

 The application of this method of Sir D. Brewster's was very 

 simple. If, for example, a diatom placed under an objective of 

 •5 N.A., and illuminated by a narrow oblique pencil, gave an 



