104 THEORY OF THE MICROSCOPE. 



illuminating apparatuses, properties being attributed to them 

 which they cannot possibly possess. 1 



Our special task is, therefore, to elucidate the theoretical prin- 

 ciples necessary for the comprehension and application of the 

 different kinds of illumination. 



1. ILLUMINATION BY TRANSMITTED LIGHT. 



The most important and only indispensable part of every illumi- 

 nating apparatus for transmitted light is the mirror. In order to 

 determine its action theoretically, let us imagine it exposed to an 

 unlimited source of light, equally luminous at all points. Every 



1 It may not be superfluous to specify a few of the views which, have been 

 published regarding the influence of the illumination on the observation of 

 microscopic details. They will illustrate how vague have been the ideas which 

 opticians and microscopists have held on this point. Schleiden says, in his 

 " Elements of Scientific Botany" ("Grundziige d. wiss. Botanik"), p. 103: 

 " It (the illuminating mirror) is made either plane or concave concave so that 

 the pencils of light emerging from it may exactly fill the aperture of the stage. 

 .... Where possible, illumination with the plane mirror is preferable ; it 

 is true, the quantity of light is not so great, but the parallelism of the rays is 

 decidedly more conducive to the certainty of the observation, for it appears that 

 displacements in the image may be caused by the convergence of the rays from 

 the concave mirror. I have often noticed this phenomenon, but I confess that 

 I can give no explanation of it, since opticians leave us here entirely in the 

 dark." Earlier authors, however, viz., Wollaston, Brewster, and Dujardin, 

 considered converging light the most favourable, and the two last-named even 

 thought that the object should lie exactly in the focus of the converging rays. 

 According to Pritchard, difficult test-objects are well shown only with diverg- 

 ing light. Harting, who mentions these opinions, explains them by the 

 different influence of objects upon the path of the rays of light, and offers on 

 his own part the eclectic view, that illumination should sometimes be effected 

 by parallel, sometimes by converging or diverging light, according to the 

 requirements of the special cases and the nature of the object. Further on, 

 when describing the means for controlling the direction of the rays, he mentions 

 that the shape of the plane mirror is immaterial, though the concave mirror 

 must of course be circular. Goring differed from him on this latter point, 

 urging that the illuminating mirror should be elliptic and not circular, it would 

 then appear circular when seen from above. He proposed a truly colossal size 

 (nearly 5 inches in length by 4 inches in breadth), with a view to increase the 

 'brightness of the image. 



We think the unprejudiced reader will be convinced, from the following 

 explanation, that the incident rays which contribute to the illumination of the 

 surf ace- elements of the field of view always converge, and that the precise 

 shape of either mirror is quite immaterial. 



