564 Transactions of the Society. 



XVII. — On the Visibilitij of Minute Objects mounted in Phos- 

 phorus, Solution of Sulj>hur, Bisulphide of Carhon, and other 

 Media. By J. W. Stephenson, Treasurer E.M.S., F.K.A.S, 



CSead 9th June, 1880.) 



The theory that there is a " loss of aperture on balsam-mounted 

 objects " was enunciated more than twenty years ago by more than 

 one writer, and although never accepted without question, it has 

 been maintained with more or less frequency until a comparatively 

 recent date, when Professor Abbe's demonstration of the theory of 

 microscopic vision rendered it absolutely untenable. 



It is not only untrue that there is a loss of aperture under such 

 circumstances, but it is positively the reverse of the truth in every 

 case in which it produces any effect whatever. 



It has already been pointed out in the Society's Journal,* 

 how this mistaken notion probably arose, viz. by failing to distin- 

 guish between a diminution of angle (which of course takes place 

 in the case of balsam-mounted objects) and a diminution of 

 aperture, two entirely different matters, as a small angle in one 

 medium (as oil) may be capable of embracing more diffraction 

 spectra than a large angle in another medium (as air), the small 

 angle having in fact the larger aperture and vice versa. 



The loss of aperture by transmitted light is therefore on objects 

 mounted in air, and this can only be prevented by mounting in 

 balsam, or some other medium which has a refractive index equal 

 to, or greater than, the numerical aperture of the immersion objec- 

 tive employed. 



This loss from " dry mounting," as it is called, arises in all 

 objectives which have an equivalent angle exceeding 180°, which 

 is the case with so many of the modern immersion objectives, and 

 notably so in those on the homogeneous principle. 



It is this fact which has induced me to bring the subject of 

 mounting in different media before the Society this evening, as it 

 is obviously of little use to obtain objectives of the large apertures 

 with which we are now familiar, if by employing them on objects 

 surrounded by air we reduce their effectiveness to the common 

 level of 180" (= 1 n. a.). 



I have said " surrounded by air " because when an object is in 

 physical contact with the cover, the loss is, by its adhesion on one 

 side, reduced to one-half, just as in an object mounted in balsam the 

 whole aperture is preserved by the contact of both its sides with 

 the medium in which it is mounted. 



But in mounting diatoms (and some other objects) in Canada 

 balsam, we find that although we have secured the full aperture of 



* See this Journal, ii. (1879) p. 774. 



