J 
24 A. C. Twining on Occuliations by the Moon and Planets 
anced by less than 4th the decrease of light. Tn this last case 
therefore the fraction of decrease is less than one-third the first. 
is exhibits an exact accordance with the following remark 
made in Sir John sarge Outlines of Astronomy, Art. 780,— 
and derived, I suppose, from a general comparison of the ordi- 
narily accepted stellar idibatiaclo with photometric results 
“the eye (in the absence of disturbing causes) does actually die 
criminate with greater precision between the relative sntonseies 
of feeble lights than of bright ones, so that the fraction 3%, for 
instance, expresses guite as great a step downwards (physiologi- 
cally speaking) from the sixth magnitude [of stars] as + does 
from the fi 
Admitting, then, that this law holds equally as true for bright 
ints seen upon a dark ground as for dark points seen upon @ 
right grdand it is easy to indicate the principle upon which the 
maximum visibility, smarts in given pregame by in- 
crease of magnifying power, can be 7 t least for dis- 
tinct objects (as in the issendnctrs and for a all focal images, 
fede affected in their distinctness by aberrations. For, in 
ab 
of unity. to nate fraction ¥ Sir 
100° 
power; Shilo, on the eae hand, the stellar ioe’ may 
toward visibility and finally acquire distinctness. Per- 
irictness, the total ae of ainsi oom ee neo con- 
