PROFESSOR MARTIN DUNCAN’S ADDRESS. 275 

Minute objects, or parts of objects, only a few multiples of the 
wave-lengths, are, however, imaged in an entirely different way, viz. 
by the diffracted rays produced by the action of the minute struc- 
ture. If ad/ 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 comformable 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 
peculiarities which are capable of producing the diffraction phe- 
nomena 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 ; 
rst, 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 ofague objects ; and 3rd, that they are not limited to 
lined or regular objects, but also extend 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 de/ineating power—the power of the Microscope to 
show things “as ¢hey are.” 
The 3rd point is, I need hardly say, most important, and one 
which it will be very interesting to have more fully elucidated, 
