Dr. North on the Angle of Aperture in Microscopes. 223 
following observation is most important. | Artists, such as painters, 
engravers, sculptors, never employ a strong light on their work. 
The most important discriminations for them, are delicate grada- 
tions of color, of shadows, or of the mingling of the two. "Thei 
eyes, according to our observation, are by natural constitution 
very easily pained and dazzled. The finest artistic effects ar 
possible only in a subdued light. An able writer on painting, re- 
‘Marks: “ Itis the property of (strong) light to convert objects into 
its own whiteness, and to take away color.” Dimness of light 
and indefiniteness of outline, assist an active imagination. Faint 
illumination in the microscope may gratify an artist’s eye by ma- 
king the representation more pictorial, and especially by assisting 
the imagination in regard to depth. Such _ pictorial effects are 
suited to the popular eye, and it is allowable for the most accurate 
and reliable observer to gratify himself and friends with them as 
exhibitions of a peculiar kind. The object of the common Ster- 
oscope, is to produce a pictorial, rather than a geometric impres- 
Sion onthe mind : yet the brief reports as yet published respecting 
Stereoscopic vision applied to the microscope, have not often dis- 
Criminated these two kinds of effect. ; 
‘he most minute objects now examined under the microscope, 
are colorless and transparent, and consequently do not need a di- 
minished illumination for bringing out delicate variations of color, 
and, as is the case with the naked eye, it may be said in general 
that the more the light, the more distinctly will ‘the minutest por- 
tions cast a shadow or reveal themselves by their refractive and 
“ispersive effect, up to that degree of general brightness of the 
image which a given eye can bear 
ck Bat what constitutes the extremest minuteness in regard to vise 
ibility 2 Simple‘and obvious as the answer is to this question, 
yet writers on the microscope speak of an increased angle as if it 
Could in all cases compensate for a higher magnifying power. 
As is the case with the telescope, or with the naked eye, an 
area is visible when it subtends an angle at the eye of at least 
“more than half a minfite of a degree.” Or, it may ‘be assumed 
that one minnte of angle is a good general measure for the visi- 
bility of areas”—*“' therefore, that areas are visible at a distance 
of about 3,000 or 4,000 times as great as their diameter.” “ But 
though such a spot can be seen, it cannot be defined as square, 
‘Cireular, &¢.” “+ 'l’o be thus clearly defined to the naked eye, 
k spots on a white ground, must have a diameter of about 
TI5 Of radius.” But black stripes can be separated when areas 
cannot be defined, and the whole is greatly dependent on illumin- 
ation. As no two persons’ eyes have precisely the same power 
of ™inute vision, the limitation in question must be to some ex- 
‘ent indefinite ; it deserves, however, and will doubtless receive 
further investigation. — 
