266 
correct representation of the structures in the case of ruled 
lines. If, now, we interpose between the objective and the 
eyepiece a diaphragm with a central apeidure only large 
enough to admit the central colourlesss image, thus obscur- 
ing all the diffracted images, and, replacing the eyepiece 
again, examine the object, all appearance of lines or other 
structure has disappeared. Now, stopping the lateral rays 
is practically reducing the aperture of the objective. .Hence, 
by reducing the aperture of an objective we diminish its 
resolving power, not as might be supposed by decreasing 
the intensity of the transmitted light, but by reducing its 
capacity to collect, or bring to a focus, the diffracted rays. 
A still more intimate relationship exists between this 
capacity to collect diffracted rays and the true appearance 
of fine structure. Since, as may be easily shown, the ap- 
pearance of structure, which is known not to exist, may be 
created by simply shutting out certain of the diffracted 
images. 
For this purpose, the same double series of ruled lines, as 
an object and the same arrangement of light as in the above 
experiment, are employed. But instead of using a diaphragm 
above the objective with a central aperture only, we employ 
one so constructed (4) as to shut out the alternate diffraction 
images of the upper row (3) formed by the coarser series of 
lines (2), but allow all those formed by the finer series to 
pass. So far as the diffraction effects are concerned, the 
results 9 S regards the two series of lines are now identical. 
What, now, is the result on replacing the eyepiece? We 
see the appearance represented in (5), the remarkable 
