156 Transactions of the Society. 



Mounting-Media of High Refractive Indices. 



To utilize the full benefit of immersion objectives, it is of course 

 essential that the object should be mounted in a medium, the 

 refractive index of which is not less than that of the immersion 

 fluid ; and down to a comparatively recent period Canada balsam 

 was most commonly used for this purpose, particularly for 

 diatoms. 



Mr. Stephenson, however, pointed out that although by the 

 use of the balsam we have attained our object so far as the aperture 

 is concerned, yet we have done so at the expense of the visi- 

 bility of the resultant image, which has become fainter by the 

 nearer approximation to equality of the refractive indices of the 

 diatomaceous silex and the balsam ; the visibility of minute struc- 

 tures being proportional to the difference between the refractive 

 indices of the object and the medium in which it is mounted. 

 Instead of balsam, therefore, media of high refractive index should 

 be employed ; thus, as the refractive indices of diatomaceous silex and 

 Canada balsam are respectively 1 '43 and 1 -54, the difference "ll 

 is the measure of the visibility of a diatom in balsam. Using a 

 solution of phosphorus in bisulphide of carbon, the refractive index 

 of which is 2 '10, the difference is '67, and the visibility of the 

 diatoms is now more than six times as great as it was in the 

 balsam. 



Continuing his researches on this subject, and endeavouring to 

 find the best media with high refractive indices, he has quite lately 

 brought before the Society the utility of an aqueous fluid capable 

 of beino- given the high refractive index of 1 • 68, viz. a solution 

 of biniodide of mercury and iodide of potassium in distilled water. 

 This more manageable and highly antiseptic medium appears 

 likely to turn out to be of great use in the observation of many 

 objects, as its strength can be diluted till the index of water is 

 obtained. This is of advantage with such objects as muscular fibre, 

 which are themselves of high refractive power, so that fluids of 

 low refractive power must be made use of to obtain the required 

 difference for more perfect visibility. The same communication 

 also contains what was much wanted, detailed practical directions 

 for mounting. 



Any one who has seen the diatoms and scales mounted in phos- 

 phorus by Mr. Stephenson's method, and exhibited at our meetings 

 during the past and present sessions, cannot fail to have been 

 struck by the great increase in their visibility as compared with 

 those mounted in balsam, or to have recognized the fact, that the 

 theoretical consideration by which their visibility was pronounced 

 to be much increased, was not unfounded. 



In addition to the increase in visibihty, there is also the fact 



