DELINEATION OF THE FINE STRUCTURE OF OBJECTS. 231 



of resolution with direct (axial) light. For the extreme cases, where 

 a and 8 are each equal to 90 and where - = 1, the minimum dis- 

 tance d is equal to half a wave-length. 



For the further consideration of the effects of diffraction the 

 results which we have above obtained may be conveniently ex- 

 pressed in a somewhat different form. We have hitherto assumed 

 the incident pencils to consist of parallel rays. We will still 

 retain this condition, only diminishing as much as possible the 

 effective source of light, by means of a small diaphragm. The 

 direct (axial) rays will then produce in the focus of the objective a 

 dioptric image of the source of light, that is, of the small dia- 

 phragm, which may be seen in the body-tube after removing the 

 eye-piece. But the deflected rays, which make with the optic 

 axis the angle a, will also produce in q and q (Fig. 125), as w r ell 

 as in every other point where interference produces light, similar 

 images, which, by homogeneous light, agree in form and size with 

 those produced direct. Any object moved between the mirror and 

 the diaphragm is clearly seen in these interference images. In white 

 light the effect is indeed so far essentially different, that the 

 diffraction images will appear coloured, because, as is well known, 

 the maxima of brightness are dependent upon the wave-length. 

 A circular diaphragm, therefore, appears in the diffraction image, 

 which is formed in the focal plane of the objective, to be drawn 

 out radially, red outwards and blue within. We mention this 

 complication incidentally, since it is apparent with the usual 

 illumination. We will confine ourselves to the case of homo- 

 geneous light, assuming the incident rays to be strictly parallel, 

 although the diffraction images are distinctly perceptible with 

 ordinary (i.e., not parallel) illumination. 



For the demonstration of the diffraction phenomena simple 

 lined objects, such as the small scales of Lcpisma saccharinum, are 

 the most suitable. The Microscope should be focused on one of 

 the scales, and a small diaphragm used ; the eye-piece is then 

 removed, and the effect observed in the upper focal plane of the 

 objective. The direct image of the diaphragm appears symmetrical, 

 and on either side are seen the coloured diffraction images at right 

 angles to the markings f of the object. If the angle of aperture of 

 the objective = 60, the larger scales will exhibit several pairs of 

 diffraction images ; the smaller ones, however, are so finely striated, 



