202 LECTURES TO SCIENCE TEACHERS. 



the shadows appear true. When you examine an object with 

 a bull's-eye condenser throwing light from one side to the 

 other, you instinctively know which way the light is coming ; 

 and when you look at an object, the object being inverted, 

 you see the lights and shadows all the wrong way. On the 

 contrary, in using these parabolic reflectors the light is thrown 

 from the opposite side, and the image being inverted by the 

 microscope, when you look at the object the lights and 

 shadows appear natural, as they would do if you looked at the 

 object without a microscope. That is certainly a great advan- 

 tage ; but there is this disadvantage in a fixed reflector, that 

 you cannot alter the angle at which the light is thrown. 

 There is, however, no reason why we should not so modify 

 these reflectors, and so arrange them with reference to the source 

 of light, as to partially overcome this objection. We have 

 also what are called Lieberkuhns, which are sections of a 

 concave silvered mirror. The light is sent up by the mirror 

 at the bottom of the microscope, and reflected back on all sides 

 of the object by this concave mirror to the object, which is thus 

 equally illuminated in all directions. There is a hole in the 

 centre of the mirror to admit the light to the object-glass, so 

 that there is no absolutely vertical light. In some cases it is 

 an advantage thus to have the light equally in all directions, 

 since it enables you to see certain structures well This is, 

 however, sometimes a disadvantage, since it gives no shadows, 

 but this could be overcome by stopping out portions of light 

 from one side or the other. With a Lieberkuhn you usually 

 do not see structures revealed by light and shade, but see 

 those due to such difference of texture as reflect light in a 

 varying manner. If the object is very small or transparent 

 you must use what is called a dark well put immediately 

 under it, so as to prevent the light passing at the side or 

 through it. With this illumination perfectly transparent 

 substances appear black, but if any part of the object be full 

 of little colourless granules they look white, and if coloured 

 the colours are well seen. These kinds of surface illumination 

 cannot be used with very great advantage, except with mode- 

 rately low powers, but for them they are often everything that 

 could be desired. 



The bull's-eye, parabolic mirror, and Lieberkuhn show objects 

 illuminated on the upper side by more or less direct reflec- 

 tion ; but another class of illuminators which like them give a 



