426 Address by the President. [Sess. 



we must turn the curved side of a plano-convex condensing- 

 lens to the lamp, but in order to render such rays parallel we 

 must turn the flane side of the lens to the lamp, and further, 

 that the lens must be so placed that the flame of the lamp is 

 in the focus of the lens. When this is done, and a piece, say, 

 of white cardboard or ground glass is held at some distance 

 from the lamp, an enlarged and inverted image of the flame 

 will be seen upon the cardboard. The size of this image is 

 conditioned by the size of the condensing-lens ; and the nearer 

 the length of the image of the flame is to the diameter of the 

 lens, so much the nearer will you have arrived at true 

 parallel rays. Another way of ascertaining when the rays 

 are parallel is to place the eye in the line of the rays and 

 to look directly at the condensing-lens. When you do so, 

 you will see one or other of the appearances shown in 

 Fig. 2, Plate XLIX., according as the lens is placed correctly 

 or incorrectly with relation to the lamp flame. A shows 

 that the flame of the lamp is correctly centred with relation 

 to the condensing-lens, but that it is without the focus of 

 the latter. B, that the flame is correctly centred, but that it 

 is within the focus of the lens, c, that the flame is in the 

 correct focus of the lens, but not in its centre. D, that the 

 flame is both correctly centred and focussed, and it is in this 

 position that the nearest approach to parallel rays is obtained. 



Another piece of apparatus for obtaining illumination for 

 opaque objects is a " Lieberkiihn," so called after the name of 

 the microscopist who invented it. It is a small silvered metal 

 cup, which fits on to the object-glass. Parallel rays of light 

 are sent up to it from the flat side of the mirror below the 

 stage of the microscope, and reflected by it down upon the 

 object. A black patch no larger than the object to be 

 viewed is placed underneath the slide to prevent light pass- 

 ing up through the object. This mode gives a very good 

 illumination, but the drawback is that the lieberkiihn can be 

 used only with the one object-glass for which it has been 

 made. 



The foregoing modes of illumination are only for object- 

 glasses of comparatively low power: they do well up to an 

 object-glass of §-inch, and, of course, it is only such object- 

 glasses which are used for the examination of opaque objects. 



