Tlie Present Limits of Vision. By Dr. Boyston-Pigott. 181 
Table of Proportionate Resolving Powers. 
Full Aperture Proportionate 
of Object-glass. Resolving Power. Semi-aperture a. 
179 sin. a = 99996 89| 
175 sin. 0 = 99905 87| 
150 sin. a = 96590 75 
120 sin. a = 86600 60 
100 sin. a = 76604 .. .. 50 
80 sin. o = 64278 40 
75 sin. o = 60870 37^ 
70 sin. a = 57360 35 
65 sin. a = 53750 32i 
60 sin. a = 50000 30 
50 sin. a = 42260 25 
40 sin. a = 34200 20 
30 sin. a = 25880 15 
20 sin. a = 17360 10 
15 sin. a = 13050 7J 
12° 38' .. .. sin. a = 11000 6° 19' 
One remarkable result of this table is that if 96,000 brilliant 
lines can be resolved with an aperture of 150°, then 11,000 lines 
per inch ought to be resolved with so low an aperture as 12° 38'. 
I should remark that the numbers given in this table are propor- 
tionate numbers, and not absolute, but they nearly correspond to 
the results of delicate observations. 
But with special adaptations to subdue or destroy the brilliant 
diffi-actions of too bright an illumination, many minute details 
before completely effaced may be brought into distinct revelation. 
Photographs taken with brilliant sunlight transmitted through 
transparent objects capable of forming brilliant lines of light and 
lens-Uke images of the sun, are necessarily subject to brilliant dif- 
fractions ; bright lens-like images are considerably enlarged, and 
look like blebs, swellings, and grotesque protuberances. At the 
same time, under proper precautions, and the use of light of the 
smallest available wave, these diffractions can be reduced. Besides 
this, the choice of object-glasses, however good they may be for 
other work, is important. Some of these producing very much less 
diffraction than others ; some giving fine and others coarse diffrac- 
tion lines. 
2. The Powers of the Eye, and Instrumentation. 
Acuteness of vision varies so considerably in different indi- 
viduals as to render an average estimate somewhat diflScult. I 
witnessed the accidental detection of Jupiter's satellites (three men- 
tioned) by a person unacquainted with their existence, and this 
person at my request drew their position on paper, which exactly 
