ILLUMINATION BY REFLECTED LIGHT. 



113 



diameter. And it follows, moreover, that an equivalent concave 

 mirror, or a Selligue prism (a rectangular prism, in which the two 

 surfaces of the right angle are convex) would yield the same results 

 as a condensing lens ; still, the latter would, in most cases, be more 

 convenient. 



The above-mentioned disadvantage, viz., that with this kind of 

 illumination the greater part of the object is within the shadow, may, 

 to a certain extent, be eliminated. For, since the distribution of 

 light and shadow in the microscopic image depends entirely upon 

 the position and aperture of the incident cone of light (assuming 

 the objective-system to be given), it is possible, by skilful applica- 

 tion of reflecting surfaces, to illuminate a larger portion of the 

 object. If, for instance, the window of the laboratory, or the 

 illuminating-lens, is the (secondary) source of light, an ordinary 

 mirror, suitably adjusted on the other side of the Microscope, will 

 duplicate the source of light. A spherical body exhibits a single 

 excentric spot of light with one-sided illumination, but will now 

 exhibit two ; and similarly a second symmetrical line is added to 

 the line of light, which is formed by a metal wire running parallel 

 to the window. If the Microscope were placed in a semicircle of 

 mirrors near the window, the effect of light would be approximately 

 as if it were exposed to unrestricted side and top light. A similar 

 effect, although not perfectly equivalent, would be produced by a 

 small concave mirror of cylin- 

 dric or parabolic section, ap- 

 plied at the end of the objective JIIIIIIllllll^^^ lii'illlBi 

 so as to focus the image of the 

 window in the object-plane. 

 To increase its utility, we might 

 give it a suitable curvature in 

 the direction of the axis, as 

 exhibited in Fig. 50. The 

 object-point p would thus be ^ IG 50 



illuminated as if exposed to 



the open sky, except the portion at the apex determined by the 

 angle q p n. 



Such an illuminating apparatus 1 has not, to our knowledge, been 



1 The apparatus is known in England as the " Silver Side-reflector," or 

 " Parabolic Side-reflector," and is mounted to slide on the front of the ob- 

 jective, like the Lieberkiihn, but with a hinged joint that facilitates the. 



I 



