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V. — On Mounting Objects in Pliospliorus, and in a Solution of 

 Biniodide of Mercury and Iodide of Potassium. By John 

 Ware Stephenson, Vice-President E.M.S.; F.K.A.S. 



{Bead 11th January, 1882.) 



In the use of modern objectives having numerical apertures ex- 

 ceeding U7iity, or, in other words, exceeding the equivalent of 180^ 

 in air, it is absolutely essential, and this cannot be too strongly 

 impressed, that the refractive index of the medium in which an 

 object is mounted, shall at least equal the numerical aperture 

 of the objective employed. 



Hence it follows that air, having a refractive index of 1, is not a 

 suitable medium in which to examine an object under an objective 

 of which the numerical aperture is more than 1, say 1"25, or 

 1 • 47 ; the former being that of the first homogeneous immersion 

 objective (made by Zeiss), and the latter that of the most recent 

 production of Powell and Lealand. 



For instance, water, having an index of 1 • 333, is a medium of 

 suflScient power to develope the full aperture of the objective of 

 N. A. 1 ■ 25 ; whilst Canada balsam, or any other medium having a 

 refractive index exceeding 1 • 47, is necessary for the latter. 



An object is literally mounted in air, only if a film of air inter- 

 vene between it and the thin glass cover. If it adhere to the 

 cover, the effect is the same as if it were half in air and half in 

 glass, and if the aperture of the objective exceed unity, its effective 



1 -{- a 

 aperture is reduced from a, to — - — , that is to say, one-half of the 



excess of aperture beyond unity, is, under these particular circum- 

 stances, entirely lost. 



The problem then is, in all cases, to find some medium fulfilling 

 the before-mentioned conditions, but at the same time such, that 

 the difference in the refractive indices of the object and medium 

 shall form a sufficiently strong image to give distinct vision, but on 

 the other hand not so great as to render the object opaque. 



In some preparations, however, the end in view is to render 

 certain parts of the object very faintly visible, in order that other 

 parts may become more visible by contrast. This is notably the 

 case in preparations which have been injected or stained with some 

 pigment, when colour alone is depended upon to depict the structure. 

 We all know that such an object in spirit or water or alcohol 

 is frequently too opaque for our purpose ; the difference in the 

 refractive indices of the material to be examined and medium em- 

 ployed is too great, and we, therefore, " clear the object," as it is 

 called, by transferring it from pure spirit, with its low refractile 



M 2 



