270 THE ELEMENTS OF MICROSCOPY. 



microscope an " instrument of precision," and its use must, 

 therefore, be undertaken under somewhat more stringent condi- 

 tions than those which apply to the concave mirror. 



In employing the latter, it is, on the whole, better to turn the 

 flat side of the flame toward the mirror. In the use of the sub- 

 stage condenser, however, the plane mirror is employed to reflect 

 the image of the edge of the flame, and by focussing the con- 

 denser the image of this edge is made to coincide with that of the 

 object. A narrow band of light is thus formed in the field, and 

 furnishes the illumination which is, of all ordinary means, the 

 most suitable and most searching for the testing of lenses, or for 

 making out the precise structure of a difficult object. The band 

 of light is not, however, suitable for the examination of objects 

 which occupy the whole, or a considerable portion, of the field of 

 view. The bull's-eye may, therefore, be employed as a parallelliser, 

 the edge of the lamp-flame being placed in its principal focus. '^ 

 The best point is found by putting the bull's-eye close to the 

 mirror in the first instance, and moving it gradually farther away 

 until the whole field is filled with light. If the centre of the 

 mirror is in a line wnth the centre of the lamp-flame, no alteration 

 will have to be made in the height of the bull's-eye as the latter is 

 moved away, but it must be exactly in the line between the two 

 and at right angles to this line. 



The above remarks apply to the use of the bull's-eye in com- 

 bination wnth some form of substage condenser, but the bull's-eye 

 may also be used either, as above described, with the concave 

 mirror or with the plane mirror. If the microscope is not fitted 

 with a plane mirror, a circular piece of silvered plate-glass of the 

 same size may be obtained from almost any glass-dealer, and if a 

 cardboard frame, large enough to fit over the concave mirror, be 

 made for it, there will be no difficulty in its use for any purpose 

 where a plane mirror is indicated. It will be easily seen that the 



* The principal focus of any lens is found by noting the distance from it 

 at which a sharp image of the sun is formed, so that the concave mirror and the 

 convex lens are alike in their effect upon the light which falls upon them, inas- 

 much as they cause parallel rays to converge, rays which converge already to 

 become more convergent, and rays which diverge to diverge less or converge, 

 according to the amount of divergence. 



