148 Transactions of the Society. 



by the difiracted rays produced by the action of the minute struc- 

 ture. If all the diffracted rays from the object are reunited and 

 reach the eye, an image of the real structure is obtained. If some 

 only of the rays are transmitted, the image is no longer necessarily 

 a true representation of the object, and the smaller the admitted 

 portion the more incomplete and dissimilar the image. Now as the 

 objects become more and more minute, the diffracted rays are more 

 widely spread, and fewer of them can be admitted by an objective 

 even of largest aperture. The visible indications of structure in such 

 images are not therefore necessarily conformable to the actual nature 

 of the object under examination, and the only inference that we are 

 entitled to draw from the image as presented to our eye, is the 

 presence, in the object, of some of the many different structural pecu- 

 liarities which are capable of producing the diffraction phenomena 

 observed in the particular case. 



Our veteran microscopist. Dr. Carpenter, C.B., has embodied, in 

 the edition of his widely known work published during 1881, a 

 statement of the leading points of the diffraction theory, which is 

 valuable as containing the results of his own matured views on the 

 subject. He says (p. 187), "This doctrine, originally based on 

 " elaborate theoretical investigations in connection with the undu- 

 " latory theory of light, has been so fully borne out by experimental 

 " inquiries instituted to test it, and is in such complete harmony 

 " with the most certain experiences of microscopists, that its truth 

 " scarcely admits of a doubt." 



There are one or two points that require to be kept prominently 

 in mind in regard to the diffraction phenomena in question ; 

 1st, that they are not to be confounded with the so-called " diffrac- 

 tion band " observed round the outlines of objects illuminated by 

 oblique light, nor with the " diffraction " rings displayed by bril- 

 liantly illuminated globules ; 2nd, that they are not confined to 

 transparent objects illuminated by transmitted light, but are also 

 produced by ojjaque objects ; and ord, that they are not limited to 

 lined or regular objects, but extend also to irregular structures or 

 isolated elements of any shape ; in fact universally, to structures of 

 all kinds, whenever the uniform propagation of the luminous waves 

 is disturbed by the interposition either of opaque or semi-opaque 

 elements, or of transparent elements of unequal refraction, which 

 give rise to unequal retardations of the waves. They therefore 

 apply not merely to the " resolving power " of objectives, but to 

 their general delineating! poicer — the power of the Microscope to 

 show things " as they are." 



The 3rd point is, I need hardly say, most important, and one 

 which it will be very interesting to have more fully elucidated, 

 having regard to Professor Abbe's statement that objects (such as 

 the flagella of Bacteria) which are only a fraction of a wave-length 



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