2 Transactions of the 



distmgiiisbmg, in transparent objects, superficial elevations from 

 depressions, that elevations appear brighter when the body of the 

 microscope is raised, whilst depressions, on the contrary, are 

 brightest when it is depressed. 



Although Mr. Stewart stated, when caUing our attention to this 

 mode of determining structure, that the idea was not new, he pro- 

 bably was the means of directing the attention of many Fellows to 

 this much-neglected method of examination. Be this, however, as 

 it may, it at once occurred to me that some medium of higher re- 

 fractive power than Canada balsam might be selected, which, whilst 

 reversing the optical effects arising from the transmission of light 

 through an object viewed in air, would not render such reversed 

 action weak by the approximately refractive equality of the object 

 observed and the medium in which it is mounted. 



To render myself more intelligible allow me to dwell for a 

 moment on the refractive indices of diatomaceous silex and some of 

 the various materials in which its condition may be analyzed. 



If diatoms are examined in air, i. e. dry, they are, in some 

 instances, too opaque for transmitted light, but on immersing them 

 in water, of which the mean index is 1 • 336, they become more 

 translucent; with media of higher refractive power the trans- 

 lucency increases until the mean index of strong sulphuric acid 

 (1*4:34) is attained, in which they become practically invisible. 

 As every object which is transparent and colourless becomes abso- 

 lutely invisible when immersed in a colourless medium identical in 

 refractive power with itself, we know approximately that the refrac- 

 tive index of diatomaceous silex is 1 " 434 (much below that of 

 quartz), and this is accordingly, for diatoms, our neutral point. 

 Although I have said colourless objects mounted in a colourless 

 medium become invisible, it is of course equally true if both are of 

 the same colour and of the same index. 



By progressively increasing the refractive power of the mount- 

 ing medium the diatoms gradually again become more and more 

 visible until, as we all know, when mounted in Canada balsam 

 (1 • 540) the coarser species are sufficiently defined for all ordinary 

 purposes; but if we require a still greater departure from the 

 neutral point or invisible condition, we must select some other sub- 

 stance of still higher refractive power. This we find in bisulphide 

 of carbon, the index of which is 1 • 678, being, I believe, the 

 highest of any known fluid. 



But, if not content with this, we may carry the bisulphide 

 higher by dissolving in it phosphorus, whose refractive index is 

 2 • 254, and thus obtain any power (with varying strength of the 

 solution) between 1 • 678 and 2 "254 ; but when such diatoms as the 

 Heliopelta are mounted in a strong solution of phosphorus, they 

 again become nearly, if not quite, as opaque as they were in air. 



