164 Transactions of the Society . 



power, to the higher one of oil of cloves, and finally into balsam ; 

 by so doing we have placed the object in a medium approximately 

 of the same index as itself, we have optically got rid of the unstained 

 portions, leaving the coloured parts more distinctly visible. 



Muscular fibre is an illustration of the effect produced on the 

 visibility of an object under these conditions. In water or glycerine 

 (optically considered) it is well shown, because the difference of 

 refractile power is sufiicient to depict the structure and not so great 

 as to obscure the view ; but mounted in Canada balsam, in which 

 the two indices are so much nearer equality (balsam being less than 

 the muscular fibre), the image is so faint that we resort to polarized 

 light, if it be necessary to examine it under such circumstances. 



This, however, is a digression from the original scope of my 

 observations, which were rather directed to the question of 

 mounting when modern objectives of large aperture are employed. 



I have pointed out that if an object adhere to the cover the 



utilized aperture is reduced to — ^ — ; but if it be mounted on the 



glass slip it is, for the purpose of our investigation, in the worst 

 possible condition, as the effective aperture is reduced to something 

 less than the equivalent 180° in air — very little less, it may be 

 perhaps, but still, if a film of air intervene, its available aperture 

 cannot be quite up to this limit. 



If Nobert's 19th band were ruled on the slide, instead of on 

 the cover, or, what is the same thing, if the plate were turned over 

 and covered with a thin glass, so that a film of air, however thin, 

 intervened, no objective that has ever been made, or I may say 

 ever will be made, would be capable of making the lines visible. 



The result would be vastly different, however, if Nobert's plate 

 were mounted in some medium giving a difference of index sufficient 

 to render the rulings visible ; such a medium is a saturated solution 

 of phosphorus in bisulphide of carbon ; here the respective indices 

 of the object and medium are, (if Nobert's lines are ruled on 

 crown glass), about 1 ' 52 and 2 • 1 ; the difference between these 

 gives a greater degree of visibility than that of a diatom in air, the 

 difference of the former being • 58, and of the latter about • 43. 



So mounted, the resolving power on such rulings would be 

 increased by more than 11 per cent, with the first homogeneous- 

 immersion objective, and by more than 19 per cent, with Powell 

 and Lealand's more recent production, so that the 19th band by no 

 means represents the attainable limit of resolution, if such rulings 

 are suitably mounted. 



In mounting objects in phosphorus there are three points of 

 vital importance : — 



1. The object must be absolutely dry, or if moistened, it must 

 be with a substance soluble in bisulphide of carbon. 



