Mercury Globules as Test Objects. By J. W. Gordon. 11 



granted, at any rate for practical purposes, that the optical axis 

 passes through the centre of the aperture of the objective. If there 

 be any discrepancy it would, no doubt, be the aperture, and not 

 the optical axis, which would determine the formation of the rings. 

 If, then, the mercury globule lies even at a very small distance out 

 of the optical centre of the objective, the Fresnel rings will be 

 visibly deformed. In one of the Microscopes exhibited this 

 evening a mercury globule is displaced slightly from the optical 

 centre of an objective. The rings, instead of forming a sym- 

 metrical concentric system, form a system in which one side is 

 very much narrowed and the opposite side expanded to such an 

 extent as to be quite unmistakable. 



To start the centring operations, therefore, the first thing to 

 be done is to place a mercury globule in the optical centre of the 

 objective. For this purpose it is well to swing the condenser clear 

 of the stage, and light the object directly from the lamp or mirror. 

 It is, moreover, convenient in all these experiments to use a circular 

 disc as the source of light, though, of course, the form of the light 

 source is of very little importance when the condenser is out of use. 

 When the observer is satisfied by the symmetrical formation of the 

 rings that the globule lies truly in the optical axis of the objective, 

 he will next proceed to rectify the position of his source of light. 

 This may be done by inclining the mirror, or if the lamp is viewed 

 without a mirror, by adjusting the position of the lamp. This 

 adjustment can be roughly made by observing the illumination of 

 the rings. If the source of light is considerably out of line with 

 the optical axis of the instrument, one part of the rings will appear 

 to be more brightly illuminated than another part. The displace- 

 ment of the light source does not very sensibly affect the form of 

 the rings or their disposition when the light source itself is at a 

 considerable distance from the stage. But it does most materially 

 affect their illumination. It is possible, therefore, in this way to- 

 obtain a collimated source of light. But a still more sensitive 

 test will be presently mentioned. 



The source of light having thus been adjusted in line with the 

 mercury globule, the condenser may next be swung into position, 

 and now the advantage of the circular source of light becomes 

 apparent. The luminous disk should be of such dimensions that 

 its image has a diameter slightly less than that of the globule. 

 When, therefore, the source of light is truly focused in the middle 

 of the field, it will be entirely occulted by the globule, and the 

 Fresnel rings will be brilliantly seen upon a dark field. If there 

 were no top lighting and no diffused illumination by reflection 

 from the surfaces of the condenser, the Fresnel rings would, under 

 these conditions, be seen on a field absolutely black ; but this 

 variously reflected light causes a considerable illumination of the 

 stage, and the Fresnel rings, therefore, are only feebly seen unless 



