ON THE THEORY OF THE MICROSCOPE. 237 



structure of known objects which are here described were observed 

 with exactly the same focussing under which the normal image 

 appeared well defined, and that they occurred under various 

 combinations of objectives and oculars with regular constancy 

 whenever the illumination was regulated in the same way. The 

 influence of the diffraction, which might be caused by the 

 diaphragm above the objective, was eliminated by means of 

 experiments made for the purpose of control. 



The partial exclusion of pencils of light entering from the 

 object, (a manipulation purposely arranged in the experiments^ 

 above described) happens unintentionally and unavoidably in the 

 ordinary use of the microscope when observing any very minute 

 microscopic structures ; for when their linear dimensions fall below 

 those of the lengths of the waves* of light, even the widest angled 

 objective cannot take in more than a small part of the numerous 

 groups of diffracted pencils at one time. This portion will be, 

 however, constantly varying according as the angular aperture 

 employed is larger or smaller, the direction of illuminating rays 

 being unchanged; or according as the illumination is changed in 

 direction, the angular aperture remaining the same. On this 

 fact rests every modification which the images of minute 

 structures undergo with altered angle of aperture or different 

 incidence of light. The constant increase of resolving power 

 resulting from oblique illumination, (in other words, the addition 

 of fresh details) and the greater ])rominence of what was before 

 visible with central illumination, is in every instance solely 

 produced either by the entrance of diffracted rays into the larger 

 aperture (with oblique illumination), which would otherwise not 

 have entered into the objective on account of their greater diver- 

 gence, or because diffraction-pencils which were but imperfectly 



* The wave length, of Red = 0.76 micro-milimeters, (signified by the Greek 

 letter fx;) that of Blue = 0.43 yu. For comparison, the distance between 

 the lines of certain test objects may be here given. Ihe longitudinal lines 

 of Hipparchia J. = 2 m, the transverse markings = 0.7 /i; Pleurosigma 

 augm., 0.48 jli ; Surirella gemma, 0.3 ^j FrustuHa saxonica, 0.25 p. 

 E 



