1882.] 



MICROSCOPICAL JOURNAL. 



101 



jective, a great variety of entirely 

 different appearances are presented 

 by one and the same object — lines at 

 a known distance apart, doubled and 

 quadrupled, — or that objects in real- 

 ity quite unlike can be made to seem 

 identical — multi-sided figures giving 

 images of squares. In short, the 

 same objects may appear to be differ- 

 ent in structure, and different objects 

 may seem to be identical entirely ac- 

 cording as their diffraction images 

 are made dissimilar or similar by 

 artificial appliances between the ob- 

 jective and eye-piece. The appear- 

 ance of particular structure can even 

 be predicted by the mathematician, 

 before it has been actually seen by 



the microscopist. 



* * * * * * * 



Our veteran microscopist. Dr. Car- 

 penter, 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 undulatory 

 " 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 ; ist, that 

 they are not to be confounded with 

 the so-called " diffraction band " ob- 

 served round the outlines of objects 

 illuminated by oblique light, nor with 

 the "diffraction rings" displayed by 

 brilliantly illuminated globules ; 2d, 

 that they are not confined to trans- 

 parent objects illuminated by trans- 

 mitted light, but are also produced 

 by opaque objects ; and 3d, 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 struc- 

 tures of all kinds, whenever the uni- 

 form propagation of the luminous 

 waves is disturbed by the interposi- 

 tion 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 power 

 — the power of the Microscope to 

 show things " as they are." 



The third 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 bac- 

 teria) which are only a fraction of a 

 wave-length in diameter, will necessa- 

 rily appear to us, not in their proper 

 proportions, but with greatly in- 

 creased diameters, and that very 

 minute striations must appear as if 

 the dark and bright interspaces were 

 nearly of equal breadth, although in 

 reality not so. 



There are obviously many histolo- 

 gical problems, such as the question 

 of the structure of muscle, which a 

 proper knowledge of this part of the 



subject may greatly help to elucidate. 

 * * * * * * * 



The Aperture of Objectives. — 



The essential difference between the 



old and the new view of aperture is 



simply, that the former considered 



only the rays which enter the objective, 



while the latter deals with those which 



emerge from it. 

 ******* 



An estimation of the emergent 

 beam, however, must obviously give 

 the same result as one of the inci- 

 dent beam (assuming them both to 

 be correctly made), it being of course 

 impossible for anything to emerge 

 that has not first been admitted. But 

 to quote Mr. Crisp : — " The great and 

 obvious advantage in dealing with the 

 emergent pencil is that it is always in 

 air, and so the perplexities are 



