Transactions of the Society. 



III. — Mercury Globules as Test Objects for the Microscope. 

 By J. W. Gordon. 



(Bead Nov. 20, 1907.) 

 Plate I. (figs. 1, 2). 



The difficulty of explaining the appearance of certain objects under 

 high magnification led me some two or three months ago to under- 

 take a comprehensive study of the appearance in the Microscope 

 of mercury globules. The hypothesis upon which I worked was 

 that the mercury globule being a simple object of known shape 

 and optical properties I could not be mistaken as to the appearances 

 which it would present, and if in any respect these appearances 

 should prove to be unexpected, they would probably be easily 

 traced to their origin. This hypothesis has not been falsified, 

 although it may be confessed that the appearance of mercury 

 globules under the conditions of high magnification has consider- 

 ably surprised me. The phenomena observed turn out to be due 

 to causes which will, I think, interest the Fellows of the Society. 

 I have therefore sought the opportunity of exhibiting some speci- 

 mens, and placing a short description before this Meeting. 



The first thing to strike the observer is a phenomenon which 

 certainly ought not to have been unexpected, although I may 

 confess that it surprised me, when I first observed it. A mercury 

 globule occupying the centre of a bright field, and illuminated by 

 a large cone of light from the condenser, presents a strong luminous 

 band about its edge, which is in fact displayed upon its under 

 face. Attention being drawn to the matter, it is quite easy to see 

 that an objective of wide angle must see for a considerable distance 

 round the under face of a spherical object. The diagram (fig. 2) 

 serves to show how this comes about and incidentally to indicate 

 the rule by which the inner edge of this luminous band may be 

 calculated. Taking the ray from the point A to the point E to be 

 an edge ray of the beam which enters the objective, it is easy to 

 see that that ray must come from a point C in the beam received 

 from the condenser, since the angle B A E must be equal to the 

 angle B A C. Moreover, if we draw the perpendicular X X r 

 through the point A parallel to the optical axis, we shall have the 

 angle E A X equal to the semi-angle of aperture of the objective. 

 This, therefore, is a known angle. In like manner the angle C A X' 



