LIMITS OF OPTICA.L CAPACITY OF THE MICROSCOPE. 427 



inner are a little wider, and the radius of the innermost bright ring 



is 1-220-* Now, as the smallest visual angle under which we can 

 a 



possibly distinguish two fine bright lines from each other may be 

 fixed at 1 minute, the figures of the brightest yellow-green light, 

 whose wave length = 0*00055 m.m., will be visible when d 

 = 1*89 m.m. Even with a somewhat larger opening the dispersion 

 of a bright point into a circle or of a bright line into a streak must 

 be noticeable. 



"When we look through such an aperture at any object which 

 shows luminous points, the difiraction figures of the separate points 

 partially cover each other, so that the fringe of dispersion circle of 

 each single point, taken by itself, may not be recognisable. The 

 effect, however, of this difi'raction, since it changes every point into 

 a small dispersion circle, obviously causes effacement of the true 

 outline, just as happens when the accommodation of the eye is 

 imperfect, in consequence of which very minute objects, which can 

 be perceived only when the image on the retina is sharply defined, 

 are unrecognisable. We may convince ourselves that this is the 

 fact by a simple experiment. The retina is most sensitively 

 impressed by such objects as gratings, consisting of alternate dark 

 and light parallel lines, whether printed on paper, ormadeof wirework 

 or drawn on glass. Let the observer place himself at such a 

 distance from the grating that, with the aid of spectacles giving 

 perfect accommodation of the eye, he may just be able to distinguish 

 the bars or lines separately from each other. Then let him place 

 before his eye a card in which fine apertures of different diameters 

 have been pierced, and observe whether he still sees the lines or 

 sees them as well with as without the card. The grating must be 

 brightly illuminated {e.ff., by exposing lines printed on paper to 

 direct sunlight), in order that the picture seen through the aperture 

 may remain sufficiently bright. On trying the experiment myself, 

 I find that a notable deterioration of the image is caused by an 

 aperture of 1.72 m.m. diameter, and the deterioration is much more 

 striking with still narrower apertures. 



