Juniata sentinel and Republican. [volume] (Mifflintown, Juniata County, Pa.) 1873-1955, 1875-06-09
SCIENTIFIC.

The Use Of the Hand as an Optical
Instriment.-Dr. F. Thomas, of Uhrdorf,
observes that, although artists are well
aware of the advantages of monocular
vision and the use of the hand as an i'm
promptu stereoscope for the inspection
of pictures, the public generally knows
nothing of them. Any one who care
fully watched the crowds that daily
thrOnged the avenues of the late Vienna
Exhibition might have seen how very,
very few persons amongst them ever
availed themselves of this ready re-
source
And vet. how different is the appear
ance of really good picture thus seen
and the same viewed in the ordinary
way by binocular visioni Regarding ii
with single eye through the hollow of
the hand as through stereoscope, We
get relief, substance, which other
wise is more or less wanting; in a word
we get the third dimension, depth
which is indispensable TO realistic effect.
Nor is the method applicable to the
contents of picture galleries alone every
photograph, every engraving and print.
of correct design, may be beneticially
treated in the same way. As with the
stereoscope, so with its impromptu sub
stitute,' We get Increased focal length,
and with it the several artistic advan-
tages thence accruing. On the other
hand, defects in drawing are ruthlessly
exposed by the same means. Trifiing
errors in perspective, which might have
passed unnoticed under ordinary cir
cunistances, stand revealed in their full
deformity.
With juster perceptions of the magni-
tude and relative dimensions Of, objects,
monocular vision, combined with the
stereoscopic use of the hand, gives us
also, a correcter appreciation of the
effects of reflected light. And this
applies not only To the confused appear
ance Occasioned by the interposition of
highly reflective media between the
object and the observer, but also TO
artificial reproductions of the same
effect.
A point ignored i'm every treatise to
which DIR. Thomas has had access is the
effec. of the hand, when thus used, in
I oGifying or correcting our perceptions
{ color. The rays of the setting sun
are flooding the landscape with golden
light. Prominent in the distance stands
forth a church tower lighted up with Il
rich orange glow. BY regarding it at
tentiyely through the hollow of the
hand, and opening and closing the latter
suitably, the tower can be made To
assume any intermediate tint betwee
the white it really is and the orange
it has assumed in the rays of the western
sun. The woods, too, dark, somber and
night-like to the unaided vision in like
ha do rn 70. no
they wore in the broad light Of MONDAY.
A bright patch on the far distance shows
a soft subdued white, and we notice
then for the first time that TO the unas-
sisted eye it presents a bright golden
color. f3't-f lif
Indeed, our conceptions of color are
mainly dependent on compariso.i-con-
trast. But these are quite inadequate
to enable us, under all circumstances, To
detect and discriminate between minor
differences of shade by ordinary unaided
binocul. vision. For that purpose, we
must have recourse to the hollow of the
hand, looking through it at the object
with one eye, and comparing the effect
observed with that produced on the
other and unshaded eye. Both eyes
may be open. "
In such cases, the chief point is not
monocular vision, but the shading OF
the eye by the hand thus applied. As
with NicOl's prism, we thus restore
the equilibrium of the blue light diff
fused through the atmospheric regions
which in the landscape above referred
to was overpowered by its complement
airy color, the orange emanating from the
sinking sun-and are so enabled TO see
objects under the hues they would
present when viewed by the white light
of noontide sun.