The Present Limits of Vision. By Dr. JRoyston-Pigott. 179 
spectra. It is also fitted with Dr. Eoyston-Pigott's searcher for 
aplanatic images, by which much greater depth of focus is attained, 
and new powers of correcting chromatic and spherical aberration, 
by moving the searcher between the objective and eye-piece." 
If then it can be clearly shown that brilliant points and lines are 
immensely enlarged by diffraction (as indeed calculated from the 
principles of the theory of light), and that the fringes thus developed 
by its undulating waves can be calculated so as to ascertain the con- 
ditions of effacements of contiguous images, it certainly follows that 
the limits of vision can be assigned for brilliant points and lines. 
But the instant the brilliant illumination ceases, these spurious 
disks vanish, and also the peculiar fringes of diffraction. An ex- 
cellent example of this is seen in a fine telescope. When a double 
star, such as e Bootes, is observed in bright twilight, I see then 
nothing but two pale snow-like disks on a milky blue sky : without 
the slightest appearance of diffraction rings. On a dark night the 
smaller star appears to fall upon a bright diffraction ring, which 
greatly obscures its appearance and the former wide space between 
the companion and its principal. The white cloud illumination, espe- 
cially from a north sky, has long been recognized as a marvellous 
improver of definition, for the very same reason — reduction of dif- 
fraction. 
These points of brilliant light are immensely enlarged into 
spurious disks even in the best telescopes (and therefore in micro- 
scopes, as often demonstrated by the writer). I may give a most 
decisive proof of this, as follows : — I found in an excellent Wray 
telescope, of 5 J inches aperture and 8 feet focal length, the disks of 
f Ursae Majoris in May last just in contact. These stars are suns 
of nearly equal magnitude, separated from each other by probably 
a thousand million of miles at the least ; yet their apparent disks 
were so much enlarged as to appear almost to touch each other, a 
thin dark line separating them. Their centres are separated by a 
little more than two semi-diameters, and these are nearly equal. 
Hence the diameters of these suns appear at least 800 millions, 
showing how enormously enlarged are the apparent spurious disks 
in my telescope above the true size. The Kev. T. W. Webb writes 
me that with his very fine 9 J " With " mirror, these stars appear 
about half a second apart. 
In a question of so general an interest as the powers of vision, 
the labours of the Jury of the Great Exhibition of 1851, in deter- 
mining microscopic power, are highly instructive. Nobert's cele- 
brated lines 11,000 to the inch were shown with a linear ampli- 
tude of 100, but lines 50,000 to the inch required a power of 
2000 ; that is, lines four and a half times closer required twenty 
times the power. This strange disproportion of power to size 
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