106 



THEORY OF THE MICROSCOPE. 



mately, when the mirror is directed to the sky the rays which 

 illuminate the field, if traced backwards to the mirror and thence 

 to the source of light, must reach this latter without diminution. 

 If, for instance, p m and p n (Fig. 44) are two marginal rays, 

 which just touch the edge of the diaphragm, they are reflected by 

 the* plane mirror A B to s and t, and by the concave mirror C D 

 to s' and t', while the central ray p o passes in both cases to r. If 

 the rays s and t meet the window in the wall //, and nothing 

 obstructs the path, all intermediate rays will proceed to the source 



of light, the sky, and in 

 an opposite direction the 

 field of view. Whether, 

 according to the form of 

 the mirror, the rays 

 diverge, converge, or 

 proceed parallel, is im- 

 material, provided a suffi- 

 ciently large portion of 

 sky affords uniform illu- 

 mination. If, however, 

 anything obstructs the 

 uniformity of the illu- 

 mination, the form and 

 position of the mirror 

 become important. If 



for instance, a part of the 



sky is more luminous 



than the adjacent portions, a concave mirror, which focuses the 

 light in p, will give a stronger illumination. Similarly, for every 

 other source of light of relatively small extent (a lamp-flame, a 

 white wall, &c.) the concave mirror will, so far as regards strength 

 of illumination, be preferable, and it should always be so adjusted 

 that the source of light is focused in the field of view. 



The size of the mirror has first to be considered in relation to 

 the aperture of the diaphragm. Let us suppose the diaphragm 

 removed; then the surface of the mirror forms the base of the 

 incident cone of light, and its aperture varies with the distance 

 of the mirror from the field of view. 



All this is, of course, founded upon the simplest laws of optics. 

 It is quite incomprehensible to us that recently-published micro- 



