The Present Limits of Vision. By Dr. Eoyston-Figott. 239 
Sir Gr. Airy deduces for mean rays where the wave-length is 
taken = \= '0000022 inch for an aperture of telescope 2 e, 
e being the radius of the object-glass, and s the angular semi- 
diameter of the given ring, or disk, viewed from the object-glass 
— the following values for e s : 
c X s = 3-70". 6-09", 8-40", for bright rings. 
e X s = 2 -70"', 5-16", 7-32", for black rings. 
And for extreme diameter of disk, where it melts into the first 
black ring, es— 2 " 76". From which it will be readily seen that 
for a lens whose aperture is 2 inches, e=l, and 2 s the diameter 
in seconds of these rings and disks, will be for a ^oint of light : 
Diameter of blight rings .. .. 7-40"; 12-18"; 16-80". 
Diameter of dark rings .. .. 5-52"; 10 •32"; 14-64". 
Extreme diameter of disk 5 • 52". 
[I have found a lens of 1 inch focus and -J inch linear aperture 
most convenient to force microscopic miniatures of the distant 
solar disk 100 inches away, as described above.] 
And since these quantities vary inversely as the size of the 
aperture, for a 5J achromatic, the actual diameter of the disk mea- 
sured up to the inner edge of the first black ring would be as 5 j 
to 2, i. e. 5 • 52" would become 
2" and ^^ih. 
The breadth or thickness of the first black ring will be half the 
difference between the diameter of the disk and the diameter of the 
first bright ring : 
7-40 
5-52 
2)1-88 
0 • 94 second per 2-inch. 
. and therefore 0-36 second for 51-inch achromatic. 
The exquisite black rings, therefore, seen by me round the double 
stars forming Castor, were actually rather less than the one-third 
of a second. The shading off, however, of the disk at its edge, 
gradually darkening into the black ring, makes this ring appear 
somewhat broader than the theoretical value above obtained, and 
may be taken approximately as half a second of arc with a 
5 J-inch achromatic linear aperture. A half-inch aperture would 
give a disk and rings above ten times larger, and more fitted for 
the microscope. 
VOL. XVI. 
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