DIFFRACTION IN MICROSCOPIC VISION. 81 



equally applies to granular bodies, or others with minute structural 

 details, but fine lines present the simplest phenomena, and are best 

 suited for the present demonstration. 



I assume a general acquaintance with the phenomena of the inter- 

 ference of light, and with the so-called diffraction images that are 

 produced when a flame (for instance) is looked at through a fine 

 grating, such as lines ruled on glass. I have, however, some grat- 

 ings for examination after the meeting, which will make the experi- 

 mental part of the subject, at any rate, very plain. 



If any, not too finely-lined, object is focussed on the stage of the 

 microscope, and the eye-piece is removed, on looking down the tube, 

 instead of seeing only a small circle of light, we see, in addition, 

 other subsidiary circles, or rather ovals (as shown in Figs. I, II), 

 which, if white light is used, appear with the colours of the spec- 

 trum, the blue being always towards the centre. 



The central circle is the direct image of the aperture in the dia- 

 phragm, and, when the eye-piece is in position, gives a view of the 

 object on an illuminated field. 



The ovals are the diffraction images (or " spectra ") of the open- 

 ing, and any two of them together will give an image of the object 

 on a dark field. They are formed by the interference of the light in 

 consequence of its passage through the object, in the same way as 

 the images of a flame are produced by a grating in the ordinary 

 diffraction apparatus. 



In the case of the scales of insects or valves of the diatoms, the 

 variations in their structure, whether arising from unevenness of 

 surface, from the unequal transparency of parts of the structure, or 

 from holes, or whatever it may be, will produce precisely the same 

 effect upon the object as is produced by the lines ruled on glass, 

 the light transmitted through such objects not entering the 

 object-glass in the same direct line which it held in its course from 

 the mirror to the object, the structure causing a number of rays to 

 be separated from the rectilinear rays on each side — these deflected 

 rays forming larger or smaller angles with the lines of direction of 

 the unchanged rays, according to the greater or less minuteness of 

 the structure. 



The theory propounded by Professor Abbe, as the result of his 

 observation of these diffraction images is, that the microscopic 

 image consists, as a general rule, of two superimposed images, each 

 distinct in origin and character, and capable of being separated 



