12 Transaction* of the Society. 



the beam from the condenser is narrowed down to a small cone. 

 It is, however, quite possible in spite of this diffused illumination, 

 to see the Fresnel rings even in the bright Held. By observing 

 them under these conditions, with the aperture of the condenser 

 opened wide, it is possible to centre the condenser in its turn. Its 

 position will, of course, be central, when the rings are again evenly 

 illuminated. 



The final centring adjustment remains to be made — that, 

 namely, which concerns the centring of the iris diaphragm. 

 This is, of course, effected in the same way as the centring of the 

 condenser. If when the iris is closed the rings are unevenly 

 illuminated, it must be moved into a fresh position until they are 

 seen to be of uniform brightness in all parts of the field. When 

 this result is reached the centring is completed and well adjusted. 

 The mercury globule being now in position, it may be employed 

 to examine the interior of the objective. For tins purpose it is 

 best to turn the sub-stage condenser aside, and to allow the light 

 from the mirror, or, better still, direct light from the lamp to illu- 

 minate the stage. It is, probably, best even for this purpose to 

 have a circular source of light, but that is not now so important 

 as when observing the Fresnel rings. The light being accurately 

 centred, if we now focus upon the principal focal plane of the 

 globule, which lies about midway between its equator and its 

 vertex, we shall see a series of images formed by the various 

 reflecting surfaces of the objective. The general form of these 

 images is that of a bright field with a circular dark object in its 

 centre, but, with a very narrow cone of incident light, such as we 



get without a condenser, the bright field may 

 lie reduced to the dimensions of a thin bright 

 outline to the dark ima^e of the globule. What 

 we actually see is a image of the stage with 

 the globule itself at its centre. The diagram 

 (fig. 4) shows generally what I take to be the 

 optical system producing these images. Here 

 one of the concave surfaces is represented by 

 the curve F F, the conjugate point to the 

 point B, the refracting surface F F being con- 

 sidered as a concave mirror, lies at point C. 

 It is brought to a shorter focus very approximately in the principal 

 plane of the globule by the upper surface of the globule, which 

 serves as a convex mirror, and operates as a field lens to shorten 

 the working distance of the concave mirror. The image so formed 

 is seen through the Microscope in the ordinary way. If the 

 point B is at the focus of the condenser, it and its conjugate 

 point C will be brilliantly luminous. But even if it be out of 

 focus, it is sure to shed light enough to be distinctly visible in the 

 dark face of the mercury globule. 



