PROCEEDINGS OF THE SOCIETY. 973 



Mr. J. W. Stephenson exhibited two slides of test diatoms 

 (Pleurosigma formosum), the one mounted in balsam and the other in 

 phosphorus, pointing out, in reply to a question, that where the 

 refractive index of the medium was approximately the same as that of 

 the object, the latter would be nearly invisible when mounted, its 

 degree of visibility depending upon the difference between its refractive 

 index and that of the medium. The refractive index of the silica of 

 the diatom was about 1 • 43, that of the balsam 1 • 64, and of the phos- 

 phorus 2'1, so that the visibility of the diatoms in phosphorus, as 

 compared with balsam, was as 67 to 11, or in other words the image 

 was six times as strong. 



Mr. Curties inquired what was the mode of mounting, and what 

 preparation was required beforehand ? 



Mr. Stephenson said that the medium was a solution of phosphorus 

 in bisulphide of carbon. Care was required owing to the extremely 

 inflammable nature of the mixtui-e ; but so very small a quantity of 

 bisulphide of carbon was used that the diatoms might be said to be 

 mounted in nearly pure phosphorus. 



Mr. Beck said that Moller had been making a series of experi- 

 ments on mounting diatoms in these highly refractive substances, and 

 he had found that, when phosphorus was used, the results were very 

 remarkable. 



Mr. Ingpen said he should like to hear from Mr. Stephenson what 

 would be the difference in the appearance of an object when mounted 

 in a medium of much greater refrangibility tban itself as compared 

 with one mounted in a medium of a much lower refractive index. 



Mr. T. C White inquired if Mr. Stephenson had employed phos- 

 phorus as a medium for mounting histological subjects ? He thought 

 the extension of its use to such objects would give very interesting 

 results. 



Mr. Stephenson said that, in addition to diatoms, the only objects 

 he had so mounted were scales of Lejnsma, Poclura, and some others. 

 The appearance presented by an object mounted in a medium of much 

 higher refractive index than itself, as contrasted with the same object 

 in a medium of lower index than itself, was in some cases very 

 distinct — for instance, the test Podura scale, which gave the well- 

 known "exclamation" markings in air, yielded little more than 

 longitudinal strite in phosphorus ; the conditions were, in fact 

 reversed. 



Mr. Crisp said that the difference in visibility between the two 

 slides was even more strikingly brought out by looking at them with 

 a low-power pocket-lens. While nothing could be seen but the out- 

 line in the case of the balsam objects, the markings of those in the 

 phosphorus were clearly distinguished. 



Mr. J. Mayall, jun., at the request of the President, explained 

 the construction of the new form of diaphragm devised by Mr. 

 Tighlmann, and adopted by Messrs. Queen and Co., of Philadelphia, 

 for Beck's vertical illuminator, and made some further remarks which 

 will be found at p. 941. 



