566 Transactions of the Society. 



silex being, as before, 1 * 43, the difference, being tbe measure of 

 visibility of a diatom in ivater, is represented by 'lO. 



The next in order is Canada balsam, with its index of 1 ' 54 ; 

 deducting the index of silex, 1 '43, we obtain the difference of •!!, 

 which is the measure of visibility of the same object in balsam, and 

 almost identical with that of water. 



The next in succession is bisulphide of carbon,* index 1*68, 

 diatomaceous silex 1 • 43, giving as the measure of visibility in 

 hisuljihide of carbon ' 25, which it will be observed is about two 

 and a half times as great as that obtainable in water or balsam. 



This result may however be exceeded by dissolving sulphur in 

 the bisulphide of carbon, although to what extent I am unable at 

 this moment to say, but as sulphur has an index of 2* 115, and is 

 moderately soluble, I think I am safe in assuming that the index 

 of the solution is 1 • 75 ; deducting from this 1 ■ 43, we obtain • 32 

 as the measure of visibility in solution of sul^jhur, which is nearly 

 three times as great as that of balsam. 



The last in the list is phosphorus, but as this, from its crystal- 

 line character, cannot be conveniently used in its solid form, it is 

 also dissolved in bisulphide of carbon, the solution being just short 

 of that point at which crystals appear. 



From the extreme inflammability of phosphorus and other diffi- 

 culties it is very improbable that it will ever be used to any great 

 extent, although there is to my mind great scientific interest in 

 the experiment. 



If we take the solution of phosphorus as having an index of 

 2*1, and deduct that of the silex, 1*43, we obtain '67 as the 

 measure of the visibility of fine diatom markings in solution of 

 2)hospJiorus, which is six times as great as that of the same object 

 in balsam, and no less than 50 per cent, higher than its visibility 

 in air itself — whilst the greater brightness of the diffraction spectra 

 will make the more refrangible rays effective, and thus give a 

 greater power of visual (as distinguished from photograjyhie) 

 resolution. 



Summarized we get the following results : — 



Table showing the Visibility of Fine Diatoms when Mounted in the 

 FOLLOWING Media, securing the full Aperture of Objective. 



Water 10 



Canada balsam 11 



Bisulphide of carbon 25 



Solution of sulphur in bisulphide of carbon . . 32 

 Solution of phosphorus in bisulphide of carbon .. 67 



The practical result of the investigation appears to be that 

 it is essential, if the whole aperture of an objective is to be 

 utilized, to mount minute structures in some medium other than 

 air. 



* Oil of cassia gives almost exactly the same result. 



