Observations on Prof. Abbe’s Haperiments. By J.W. Stephenson. 83 
and other things, are not simply direct dioptrical images, such as 
the mere outline of an object, but are the result, in most cases, of 
the combination, or fusion together, of the central pencil with 
certain secondary images, produced by the interference of those 
pencils of light, into which, by diffraction, the incident beam of 
light is, in passing through the object itself, resolved; in other 
words, that the principal or central beam of light alone is not 
sufficient truly to depict fine lines, small apertures, or other minute 
structural details, but that, as far as resolution is concerned, two 
or more pencils are always necessary to produce the desired effect. 
These pencils may, or may not, include the principal or dioptric 
beam, but where the latter is excluded the image necessarily 
appears on a dark field. 
. Further, his contention is, that when from any cause whatever, 
whether from the angles formed by the intersection of lines or 
the closeness of the lines themselves, whether from the aperture of 
the object-glass, or when, by artificial means, the diffraction images, 
as seen within the body of the microscope, are made similar, the 
microscopic images themselves will be identical. 
The diffraction images of a lined object, in focus on the 
stage of the microscope, may readily be seen by removing the 
eye-piece and looking down the tube-of the instrument. Here, 
with the light central, and the lines on the object parallel, the 
coloured spectra are distinctly visible, going off on either side at 
right angles to the direction of the striz, the most refrangible rays 
next to the central beam of light. The latter fact is particularly 
mentioned, as it has an important bearing on the limits of visi- 
bility and on the photographic reproduction of microscopic objects. 
Professor Abbe has supported his views by some very striking 
experiments, which appear to me to be a complete practical demon- 
stration of the truth of his mathematical deductions; and by his 
permission, I propose to exhibit under the microscope this evening 
four or five of those which impress me as being the most important, 
and therefore the most interesting. 
[st Experiment.— The purport of the first experiment is to 
illustrate the production of identical microscopic images by different 
structures, when, by artificial means, the diffraction pencils arising 
therefrom are made similar in number and position, within the tube 
of the instrument, as previously mentioned. | 
This experiment 1s made on a grating formed of alternately 
long and short parallel lines (Fig. 1), ruled with a diamond through 
a film of silver, of extreme tenuity, deposited on the under side of 
a thin glass cover, and subsequently cemented with balsam to an 
ordinary glass slip, the coarser lines being about 1790 to the inch, 
and the finer about 3580. 
This grating gives rise to two sets of diffraction spectra, when 
