DIFFRACTION IN MICROSCOPIC VISION. 85 



have one in which the lines are oblique — a purely illusory 

 appearance. 



It results from similar experiments with these diffraction images 

 that it is not possible to determine the real structure of P. angulatum 

 merely from its image as presented by the microscope. With an object- 

 glass of large angle, it appears, as is well known, to be covered with 

 hexagonal markings, and when the diffraction spectra are examined, 

 they are seen to be six in number, ranged round the margin of the 

 field at equal distances, with the dioptric beam in the centre. 



These hexagons are not, however, necessarily a true appearance, 

 for some entirely dissimilar objects may be made to appear with 

 hexagonal markings by cutting off all the spectra except the six 

 just mentioned. Thus a set of lines, which, crossing one another at 

 an angle of 60°, form in reality rhombic spaces, presents two rings 

 of spectra — the inner one identical in position with that of P. angu- 

 latum. The rhombic spaces may be made to appear hexagonal, 

 simply by reducing the aperture by a diaphragm, so as to exclude 

 the outer and include only the inner ring. Or, again, by varying 

 the combinations of the spectra, different sets of hexagons of varying 

 size and position may be produced, all of which cannot, of course, 

 represent the true structure. 



It is thus seen that dissimilar structures give identical microscopic 

 images, when the difference of their cliffractive effects in the micro- 

 scope is removed, and that, conversely, similar structures give dis- 

 similar images, when their diffractive effects are made dissimilar, the 

 moral of the whole being, as Prof. Abbe puts it, that minute 

 structural details (as stria, &c.) are not, as a rule, imaged by the 

 microscope, geometrically or dioptrically, in accordance with the real 

 detail of the object, and cannot be interpreted as morphological, but 

 only as physical characters — not as images of material forms, but as 

 signs of material differences of composition of the particles com- 

 posing the object ; so that nothing more can safely be inferred from 

 the image as presented to the eye than the presence in the object of 

 such structural peculiarities as will produce the particular diffrac- 

 tion phenomena on which the images depend. 



In conclusion, I may mention that we are favoured to-night by 

 the presence of Mr. Stephenson, the Treasurer of the Royal Micro- 

 scopical Society, who first drew the attention of that Society to 

 Professor Abbe's theory, and who has kindly come to exhibit, 

 under the new oil-immersion objective (made on his suggestion at 



