Mercury Globules as Test Objects. By J. W. Gordon. 7 



is the semi-angle of aperture of the beam received from the con- 

 denser. This is not necessarily a known angle, but if it can be 

 ascertained, it is obvious from the diagram that the angle at 0, the 

 centre of the globule, subtended by the illuminated band A G, is 

 equal to half the sum of the angles E A X and C A X'. For writing 

 u and % for these angles respectively, we have 



ZAOE=Z^-ZAEO = ? A±^>-(|-^ = i(^+%) [1] 



This bright belt is clearly seen in the photograph (Plate I. fig. 1) 

 of a mercury globule exhibited under these conditions. In addition 

 there is seen in the centre of the globule a bright spot of light 

 reflected from its upper face. The light which thus reaches the 



Fig.|2. 



upper face of the globule can only come, and does in fact come, 

 from the lenses of the Microscope, which reflect back and condense 

 upon an object placed in the middle of a field a very strong light 

 received by them from the field. In the photograph this spot of 

 light is seen unfocused since it occupies a position about midway 

 between the vertex of the globule and its equatorial plane, and the 

 Microscope for the purpose of taking this photograph was, in fact, 

 focused upon the illuminated belt which lies immediately below 

 the equatorial plane upon the under face of the globule. But, 

 by focusing up to the principal focal plane of the globule, it being 

 considered for this purpose as a convex mirror, a detailed view may 

 be obtained of the interior of the Microscope. 



If a mercury globule upon the stage of the Microscope is 

 illuminated by light from a very small source of illumination, and 

 if, further, the condenser is so disposed that the image formed by 

 it of the source of light lies a little above the equatorial plane of 



