Mercury Globules as Test Objects. By J. W. Gordon. 9 



wide-angled lens. The silex of angulation has a deep brownish 

 yellow colour, which may be seen when the specimen is viewed by 

 transmitted light, as, for example, by means of an objective of low 

 angle. The coloration then is seen to be very strong, but if the 

 same specimen be viewed while illuminated from the same source 

 of light through a wide angled immersion lens, the yellow colour 

 will entirely disappear. The silex then appears to be of a brilliant 

 white, and detail which by the transmitted light was wholly 

 invisible comes strongly into view. This is, I think, undoubtedly 

 a case of top lighting, and the distinctive image which a wide- 

 angled lens alone can show is to be attributed to the illumination 

 of the upper surface by top light from the objective. 



Another very familiar instance of the effect of this top lighting 

 is afforded by the much discussed phenomenon known as an 

 unoccupied aperture. The top light from an immersion objective is 

 given back in very great abundance from its peripheral zones. 

 This may seem to be a natural thing if one considers only that the 

 peripheral zones comprise a large proportion of the whole surface. 

 But there is probably some reason which I have not been able to 

 divine, for assigning to the peripheral zones a reflecting power more 

 than proportional to their area. For if the flooding of the stage with 

 this top light be watched while the observer cautiously opens the 

 iris diaphragm, it will be seen that nothing particular happens 

 untii a certain point is reached in the expansion of the condenser 

 aperture. At that point the top light comes rushing in, and rapidly 

 spreads over the field. If any reflecting surface lies between the 

 object and the objective, the image is almost instantaneously 

 ruined, and all detail is blotted out in a blazing mist of diffused 

 illumination. It will now, I think, be evident why the explanation 

 of the phenomena connected with the unoccupied aperture has 

 given so much trouble to microscopists. They have omitted to 

 consider the great abundance in which the peripheral zones 

 supply this top light, and they have therefore omitted also to 

 consider how all important it is to the use of an immersion 

 objective that the space between the specimen and the first 

 reflecting surface should be filled by an absolutely non-reflecting 

 medium. When the front lens of the objective and the cover 

 glass have different refractive indices, or when the oil interposed 

 between them has a refracting index differing, it may be only 

 slightly, from theirs, there is, of course, a reflecting plane or more 

 than one, interposed between the specimen and the first refracting 

 surface. The same thing occurs of necessity in the case of all 

 specimens which are mounted dry. When from either of these 

 causes such a reflecting surface exists it will, when illuminated by 

 the top light from the objective, interpose an obstacle through 

 which it is quite impossible to see anything except the most 

 strongly marked features of an object. It is therefore not 



