Observations on Prof. Abbe’s Kxperiments. By J.W. Stephenson. 87 
of spectra, the structure of such an object as that under consideration 
is absolutely indeterminate. 
In this experiment the slit diaphragms are entirely discarded, 
and the crossed grating is examined with a simple circular stop, 
which is used merely for the purpose of so reducing the angle of 
aperture that the first row of spectra only shall be admitted. 
The illumination is central, and an examination of the interior 
of the tube shows seven pencils of light, the bright dioptric beam 
being in the centre of the field, with six equidistant spectral rays 
around the margin. 
Let it now be clearly borne in mind that we are about to 
examine a structure which we know to be entirely composed of 
distinct rhombic markings. 
On replacing the eye-piece for this purpose, we see hexagonal 
markings over the entire field, as in Plewrosigma angulatum, and 
this effect has been produced by simply so reducing the aperture 
relatively to the fineness of the object, that the first spectra only 
are admitted. 
From this microscopic image we can infer nothing as to the 
real structure of the object under examination; we know it to be 
rhombic, but it appears to be hexagonal. 
But the bright central beam and six coloured spectra which 
have produced this result, are identical in aspect with that pre- 
sented by a single valve of P. angulatum with central light. 
Compare Fig. 12 and the inner ring of Fig. 14. 
This diatom with central light, under the highest powers and 
with the largest apertures, necessarily presents the same spectral 
appearance, in consequence of the fineness of the striz, or holes 
(whichever they may be), the dispersion being too great to admit 
the second row of spectra. 
It has now been proved that, with the means employed, no 
definite inference could be drawn of the real structure of the arti- 
ficial object, and it is equally certain that this demonstration will 
apply with equal force to the valve of P. angulatum, the hexagonal 
markings of which may, to use the words of Professor Abbe, arise 
from “two sets of lines, or three sets of lines or isolated apertures 
of any shape in the object itself.” 
If it were possible to admit the second row of spectra, a nearer 
approach to a knowledge of the true structure would be obtained, 
as the larger the number of diffracted rays admitted, the greater 
the similarity between the image and the object, the keystone 
of the theory being that “the interference of atu the diffracted 
pencils, which come from the olject, produces a copy of the real 
structure,’ as in a dioptrical image; but this, as has been abun- 
dantly shown, is rendered impossible by the great dispersive power 
of many fine structures. 
