296 
Transactions of the 
("Monthly Microscopical 
L Journal, Dec. 1, 1869. 
sible. In the case of double stars visible with Mr. Browning's 
8-|-incb silvered specula (on favourable nights with 400 diameters), 
the closest I believe is Gamma ^ Andromedae, to seconds apart; this 
interval with 400 amounts to a visual angle of 400 x to" = 160", 
the smallest perhaps under which the eye can effectively divide and 
separate, not merely elongate, fine double stars ; and but for the 
sparkling glory of these suns, so minute and dwindled in an abyss 
of distance, doubtless much closer stars (with instruments more per- 
fectly free from aberration) should become visible. Only the finest 
known definition can sever those glittering points. 
The healthy human eye, then, consisting of an achromatic com- 
bination of refracting media, and of a sensitive recipient of rays 
whose sensibility is limited to the minuteness of the distribution of 
its nerves, differs widely in its powers of definition, and particularly 
in its qualification of distinguishing a line from a string of dots 
each subtending an angle less than 2'. 
The subject of definition is beautifully illustrated by the whole 
range of what are termed line test-objects, whether as the various 
pleurosigmata, striated scales, or artificial lines on glass, as Nobert's ; 
the latter, however, being grooves cut or ploughed into glass by a 
fine, pointed diamond, cannot offer the same characteristics for defi- 
nition as objects whose lines are caused by small, spherical bodies 
raised in relief, the complete resolution of which requires besides 
definition " penetration " (or less angular aperture than is necessary 
to catch the shadows arranged lineally upon glass). For these and 
other reasons the distinct definition and portraiture upon the retina 
of minute illuminated or shaded spherical dots, of small visual angle, 
placed in close contact, may be regarded as the severest known 
standard test both for the performance of microscopes and telescopes. 
The disadvantages of deepening the eye-pieces in place of exalt- 
ing the magnifying power of the object-glass or reflector, are at once 
so appreciable, in the appearance of haze, cloudy definition, and 
obscurity, caused by the increase of the " least circle of confusion," 
and the destruction of ajolanatism, that great genius has been 
displayed in constructing object-glasses from T^th to the ^sth and 
■sVth of an inch focus, whose performance has almost been regarded 
as beyond mere praise, entitling them to the highest honours which 
nations in their art exhibitions can confer. Yet in the best glasses 
there is a certain residuary aberration (chiefly spherical) which 
obscures the clear definition under a power of 1000* of a string of 
* To find the visual angle (0) of the 80,000th of an inch under a power of 
1000 at 10 inches distance (the usual focal distance of the last image from the 
eye of the observer), 
1 
^. , perpendicular ^^^^ ^ 80000 _ 1 
Sine e = = — ^ • 
hypothenuse 10 
Hence 6 the visual angle = 4-j-3_ minutes nearly, or nearly double the visual 
angle of a two-foot bull's-eye seen at 1000 yards target. 
