176 Transactions of the Royal Microscopical Society. 
with the telescope. Now whether the point of hght is viewed or a 
series of points forming a continuous Hne, as a poHshed rod of 
metal or glass in sunshine, diffraction effects are equally produced. 
And as a rule few eyes are capable of separating double disks 
or lines of light whose dividing interval is less than a minute of 
arc. But if the disks are obtained by solar light, the instant the 
sunshine is obscured by a white cloud, fine details much more 
minute than the double disks or bright lines become plainly 
visible: the diffraction had thus obscured objects much smaller 
than the fringes — a most significant fact. 
Example. Suppose a highly finished brass instrument is 
placed at a distance and viewed with a high power by an excellent 
telescope. At several points in bright sunshine brilliant disks and 
lines of light surrounded by a black border and continuous diffrac- 
tion lines will be readily seen. A variety of objects placed together, 
as black glass beads, thermometer bulbs, and other shining surfaces, 
give by comparison numerous diffraction appearances ; two polished 
needles placed very close together, reflecting threads of light in 
sunshine, become blended and invisible from these diffractions. 
But the moment the sun goes behind a cloud, the details become 
instantly visible. Precisely the same effects are produced by 
forming miniatures of such objects in sunshine and then viewing 
these miniatures with a high-power microscope. For this purpose 
gold leaf pressed between a glass cover and slide displays beautiful 
fringes, and especially the black interference ring, under transmitted 
solar illumination. 
Supposing then that the eye of the observer is so keen as to 
be able to separate two lines of bright light subtending a visual 
angle of one minute in the field of view, the instant the illumina- 
tion of sunshine ceases, minute details of the objects become visible 
which had been totally hidden by the spurious disks and enlarge- 
ments of brilliant diffractions. 
Experiment 1. — An optical gauge containing seven standard 
double convex lenses having their focal lengths respectively 
1 inch, f, i, I, i, -1, i, 
was placed in sunshine, and in such a position between the tele- 
scope and the house either that brilliant disks could be observed 
by reflected images of the sun, or under passing clouds the beau- 
tiful details of the miniature house * of a very light colour could 
equally be examined. During bright sunshine nothing but the 
spurious disk and companion rings of diffraction could be descried. 
* This object, as being white and relieved with dark-looking windows, 
chimneys, and a background of lofty trees, gave a beautifully minute picture, 
displayed by the half-inch lens about 60 feet distant. 
