112 THEORY OF THE MICROSCOPE. 



overcome by the application of a condenser (L L in the Fig.), or a 

 concave mirror, which will thus render exactly the same service as 

 in the case of transmitted light. All intermediate rays reach the 

 source of light, and consequently, in the opposite direction, the 

 objective. The point p is thus illuminated by a complete cone of 

 light, having an aperture equal to that of the objective. 



Let us now suppose similar cones of light drawn to all points to 

 the right and left of p ; obviously a part only will reach the source 

 of light, whilst some are reflected either towards the objective, or 

 towards points of the surface of the metal-work. The cones 

 are, therefore, only partially formed. The loss is greater the nearer 

 we approach, on the one side, the vertex o, and on the other side, 

 the edge of the hemisphere. In all cases, complete obscurity 

 must be produced at o, since the incident cone' of light coincides 

 with the reflected one ; similarly for the point r, situated on the 

 right of p, illumination is, under the given conditions, quite impos- 

 sible, for all rays reflected 

 a ^ r are incident beyond 

 the effective portion of the 

 objective. The hemisphere, 

 therefore, appears bright only 

 in the part surrounding p 



F (Fig. 49, a), and even if the 



source of light were equally 

 intense at all points equidistant from o which is not practically 

 the case, as the light is incident from the side this bright spot 

 would appear only as a narrow circular zone (Fig. 49, &). 



We will endeavour later on, when discussing the Theory of 

 Microscopic Observation, to determine more exactly the distribu- 

 tion of light and shadow for certain object-forms ; we shall here 

 merely show that an illumination of this kind cannot, in general, 

 be favourable for the determination of the form-relations of a given 

 object, for even with a moderate aperture of the objective, a large 

 portion of the surface is in the true shadow, or in the penumbra. 

 As a rule, it is only applicable when the amplification is not more 

 than 100 120 linear, so that the aperture of the incident cone 

 (k p I in Fig. 48) is considerably larger than that of the objective. 

 The application of a condenser enables us to produce such a cone, 

 even if the Microscope is some distance from the window, provided 

 the focal length of the lens is not too great in proportion to its 



