1S8 PROCEEDINGS OF THE ACADEMY OF [March, 



Diaphragm may then be moved slightly and the diffracted beams 

 again recorded on same drawing. This can be repeated until the illu- 

 mination from the pinhole has traversed every portion of the objective's 

 aperture, or until the patience of the observer is exhausted, when, if the 

 diaphragm be removed and the objective fully illuminated, it will be 

 absolutely certain that all the diffracted beams represented on the 

 composite sketch will still be present within the aperture of the objec- 

 tive. Twenty years ago Abbe published a paper dealing specially 

 with fully illuminated apertures. 



The other experiment described, which states that the image of an 

 Abbe diffraction plate, illuminated through a narrow central aperture, 

 and examined under a one-inch objective, becomes invisible when 

 sunlight is used as the source of illumination, would be equally falla- 

 cious if true, as the glare of such light might prevent any image being 

 seen, but its presence could be demonstrated by receiving it on a ground 

 glass screen or by photographing it. But even the facts are here mis- 

 stated. I did not doubt that the image might be obscured by the 

 blinding effect of direct sunlight, and merely carried out the experi- 

 ment to demonstrate that the image would appear on the screen of a 

 camera, which from photomicrographic experience I knew would be 

 the case ; but I found not only that the lines were well defined on the 

 ground glass of the camera, but also were equally distinct when exam- 

 ined directly through the microscope. Although Wright's description 

 of the experiment had been followed in every detail, the cemditions 

 were likewise varied in every possible way that might lead to the 

 results claimed. A shade prevented the sunlight from reaching any part 

 of the microscope except the mirror, but in addition it was wrapped 

 in several folds of a heavy focussing cloth, preventing the possible 

 entrance of any light save the narrow central beam, which was modified 

 in size first with the iris and then by a specially made pinhole 

 diaphragm, much smaller than could be secured with the iris, or with 

 the diaphragm accompanying the Abbe diffraction apparatus, recom- 

 mended by Wright for the purpose. The disk of the sun was focussed 

 sharply in the plane of the lines and also above and below; the con- 

 denser removed and sunlight merely restricted by the minute and 

 larger diaphragms tried; eyepieces from two-inch to one-fifth inch 

 employed; the fine one-inch objective at first used remove el and 

 replaced by the poorest one of this power in my collection, a cheap 

 single system affair of low aperture; but it was useless to change the 

 conditions, as under any of them the image remained just as sharp 

 anel distinct as it would have been with lamplight, and the only result 



