On the Visibility, &g. By J. W. Stephenson. 565 



the objective, and therefore its fall resolving power, we have done 

 so at the expense of the visibility of the resultant image, which 

 has become fainter by the nearer approximation to equality of the 

 refractive indices of the diatomaceous silex and the Canada balsam 

 in which the object is mounted ; the markings, whatever they may 

 be, are less pronounced than they would have been in air had the 

 structure been sufficiently coarse for resolution in that medium, a 

 result which Professor Abbe has shown to be attributable to the 

 paler diffraction spectra yielded by the balsam-mounted object — 

 hence we see that it may be possible to resolve an object in balsam 

 which would be impossible in air, but that if resolvable hi hoih it 

 would be more visible in air than in balsam. 



It may be demonstrated that the visibiHty of very minute 

 structures is ijroportional to the difference between the refractive 

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

 {n-ni). 



It follows from this that when this diiference = 0, or is very 

 small, the structure is invisible. This is the case, as most of us 

 know, when diatoms are immersed in strong suljihuric acid, and it 

 may therefore be inferred, as was pointed out some years ago, that 

 the refractive index of diatomaceous silex is about 1 • 43, which, 

 without any pretence that it is exact, I shall assume as its true 

 value in the following observations. 



As the visibility of minute structures is proportional to the 

 difference between the refractive indices of object and medium, it is 

 necessary to give a short table of the refractive indices of those 

 substances to which I shall refer, and fi'om which the differences of 

 the indices are to be deduced. 



Table of Indices. 



Air =1 



Water = 1-33 



Diatomaceous silex and sulphuric acid = 1'43 



Canada balsitni = 1'54 



Bisiilpiiide of carbon = l'G8 



Solution of sulphur in bisulphide of carbon (approximately) . . = 1-75 



Sulphur = 2-11 



Solution of phosphorus in bisulphide of carbon (approximately) =2 10 



The first case we will consider is that of the visibihty of a diatom 

 in air, which, although it is otherwise excluded from consideration 

 in consequence of the lo.-^s of aperture involved, is nevertheless 

 valuable as a standard of comparison. 



The index of diatomaceous silex being taken as 1 • 43, and 

 that of air being 1, we have as a measure of the visibility of a fine 

 diatom in air the number "43. 



Taking now the various media in succession, and connucncing 

 with water, of which the index is I '33, the index of diatomucious 



