Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
1138 VISION. 
absorbs almost all colours except red, which 
it reflects ; but those substances which reflect 
all the rays appear white ; those which ab¬ 
sorb all are black. The brilliancy of tints 
is greatly increased when viewed in light of 
their own colour, as may be proved by throw¬ 
ing the red rays from a prism upon any scarlet 
object, or the green rays upon a green leaf. 
The colour of transparent substances depends 
upon their property of absorbing some of the 
colours of white light, and transmitting others. 
The blue tint of the atmosphere in reflected 
light, and its red morning and evening tinge, 
are to be ascribed to this cause. 
Dyes and paints are substances which, 
when applied to bodies, so change their sur¬ 
faces, that when seen in white light, they re¬ 
flect only the particular colour of the dye or 
paint. There are several modes by which 
white light can be reproduced, of which a 
simple one is, the rapid rotation of a disk 
painted in stripes, with the prismatic colours 
in the correct proportions. In this case, the 
eye receives the impression at the same time, 
and in the same place, of a red circle, an 
orange, a yellow circle, and so on, and conse¬ 
quently a white circle, since the sensation of 
white is but the simultaneous sensation of all 
these colours. 
As in the production of white light, it is 
necessary that all the simple colours should 
exist in their due proportions; so it is evident 
that by suppressing or increasing one the 
harmony will be destroyed, and the light will 
be no longer white. Thus, for instance, by 
suppressing red, we obtain a blneish green, 
which, compounded with red, would form 
white light. Whenever two colours, simple 
or compound, fulfil this condition, they are 
said to be complementary one to the other. 
They are as follows : — 
Red - 
- Blueish-green. 
- Blue. 
- Indigo. 
Green - 
- Violet-reddish. 
Blue - 
- Orange-red. 
Indigo - 
- Orange-yellow. 
Violet - 
- Yellow-green. 
Black - 
- White. 
White - 
- Black. 
With respect to the production of light, 
bodies are divided into luminous and non- 
luminous ; among natural bodies some pos¬ 
sess in themselves the property of exciting 
in our eyes the sensation of brightness, or 
light, as the sun and other heavenly bodies 
which shine bv their own light. lhere is 
also chemical light, or that produced by com¬ 
bustion, electric light, phosphorescent light, et 
cetera. Non-luminous bodies are such as 
become visible only when light falls upon 
them from some luminous source. 
Bodies are also divided into transparent 
and opaque, in reference to their capacity for 
transmitting light through their substance, 
though this property depends, not merely on 
their absolute transparency, but also on the 
density of the medium through which the 
light passes. There is no perfectly opaque 
or perfectly transparent substance known. 
Diamond is nothing more than charcoal in a 
different state of molecular aggregation, and 
gold can be made pervious to light. On the 
other hand, the purest air or clearest water 
gradually extinguishes by absorption the rays 
transmitted through them. According to 
Bouguer the purest sea water loses all its 
transparency at a depth of 730 feet, and the 
reason that more stars are visible from the 
summit of a lofty mountain than from the 
level of the sea is, because the light from the 
more distant stars becomes so much enfeebled 
during its passage through the lower strata of 
the atmosphere, that it has not sufficient 
power to affect the sight. 
If a pencil of rays diverging from a lumi¬ 
nous point fall upon the surface of a convex 
lens, they will not all be equally refracted. 
The ray which passes through the axis of the 
lens will not be changed in its course, but the 
remainder of the rays will be more and more 
refracted in proportion as they recede from 
the optical centre of the lens. When the 
rays pass out from a bi-convex lens into air 
they are refracted from a line perpendicular 
to the point of emergence : the effect is to 
cause them all to converge towards the cen¬ 
tral ray to a point at which they meet, called 
the focus. The distance between the focus 
and the refracting surface is the focal dis¬ 
tance or focal length, and is influenced by the 
refracting power of the lens, the amount of 
its curvature, and the distance of the lumi¬ 
nous body. 
Parallel rays entering any plano-convex or 
double convex lens at an equal distance from 
its axis, are concentrated to the same focal 
point, but as the peripheral rays are more 
refracted than the central rays, they are sooner 
brought to a focus ; hence the image formed 
at the focus of the lens is somewhat indistinct 
at its edges. This imperfection is due to 
what is termed spherical aberration, and is 
counteracted either by shutting out the peri¬ 
pheral rays, or by such a combination of lenses 
as will establish a just proportion between the 
refraction of the central and peripheral rays. 
Such lenses are made of crown glass, com¬ 
posed of flint and alkali only, and flint glass, 
in which oxide of lead is added to the other 
materials. The latter possesses a much higher 
dispersive power than the crown glass ; but 
the refraction of the rays is nearly the same 
in both, and when combined, achromatic lenses 
are obtained. This term is applied from their 
utility in obviating another source of con¬ 
fusion, chromatic aberration, which is caused 
by the unequal refrangibility of the prismatic 
rays when transmitted through an ordinary 
lens, whereby the images are fringed with 
colours, and are rendered even more indis¬ 
tinct than by spherical aberration. Newton 
supposed that an achromatic lens was an im¬ 
possibility ; but in 1757 Dollond completely 
succeeded in overcoming the difficulty by the


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