reflect; neither are they red, because green bodies absorb most of the
red rays. But though bodies, from the arrangement of their particles,
have a tendency to absorb some rays, and reflect others, yet it is not
natural to suppose, that bodies are so perfectly uniform in their
arrangement, as to reflect only pure rays of one colour, and perfectly
to absorb the others; it is found, on the contrary, that a body
reflects, in great abundance, the rays which determine its colour, and
the others in a greater or less degree, in proportion as they are nearer
to or further from its own colour, in the order of refrangibility. The
green leaves of the rose, therefore, will reflect a few of the red rays,
which, blended with their natural blackness, give them that brown tinge:
if they reflected none of the red rays, they would appear perfectly
black. Now I shall hold the rose in the blue rays----
_Caroline._ Oh, Emily, Mrs. B. is right! look at the rose: it is no
longer red, but of a dingy blue colour.
_Emily._ This is the most wonderful, of any thing we have yet learnt.
But, Mrs. B., what is the reason that the green leaves, are of a
brighter blue than the rose?
_Mrs. B._ The green leaves reflect both blue and yellow rays, which
produce a green colour. They are now in a coloured ray, which they have
a tendency to reflect; they, therefore, reflect more of the blue rays
than the rose, (which naturally absorbs that colour,) and will, of
course, appear of a brighter blue.
_Emily._ Yet, in passing the rose through the different colours of the
spectrum, the flower takes them more readily than the leaves.
_Mrs. B._ Because the flower is of a paler hue. Bodies which reflect all
the rays, are white; those which absorb them all, are black: between
these extremes, bodies appear lighter or darker, in proportion to the
quantity of rays they reflect or absorb. This rose is of a pale red; it
approaches nearer to white than to black, and therefore, reflects rays,
more abundantly than it absorbs them.
_Emily._ But if a rose has so strong a tendency to reflect rays, I
should imagine that it would be of a deep red colour.
_Mrs. B._ I mean to say, that it has a general tendency to reflect rays.
Pale coloured bodies, reflect all the coloured rays to a certain degree,
their paleness, being an approach towards whiteness: but they reflect
one colour more than the rest: this predominates over the white, and
determines the colour of the body. Since, then, bodies of a pale colour,
in some degree reflect all the rays of light, in passing through the
various colours of the spectrum, they will reflect them all, with
tolerable brilliancy; but will appear most vivid, in the ray of their
natural colour. The green leaves, on the contrary, are of a dark colour,
bearing a stronger resemblance to black, than to white; they have,
therefore, a greater tendency to absorb, than to reflect rays; and
reflecting very few of any, but the blue, and yellow rays, they will
appear dingy, in passing through the other colours of the spectrum.
_Caroline._ They must, however, reflect great quantities of the green
rays, to produce so deep a colour.
_Mrs. B._ Deepness or darkness of colour, proceeds rather from a
deficiency, than an abundance of reflected rays. Remember, that if
bodies reflected none of the rays, they would be black; and if a body
reflects only a few green rays, it will appear of a dark green; it is
the brightness, and intensity of the colour, which show that a great
quantity of rays are reflected.
_Emily._ A white body, then, which reflects all the rays, will appear
equally bright in all the colours of the spectrum.
_Mrs. B._ Certainly. And this is easily proved by passing a sheet of
white paper, through the rays of the spectrum.
White, you perceive, results from a body reflecting all the rays which
fall upon it; black, is produced, when they are all absorbed; and
colour, arises from a body possessing the power to decompose the solar
ray, by absorbing some parts, and reflecting others.
_Caroline._ What is the reason that articles which are blue, often
appear green, by candle-light?
_Mrs. B._ The light of a candle, is not of so pure a white as that of
the sun: it has a yellowish tinge, and when refracted by the prism, the
yellow rays predominate; and blue bodies reflect some of the yellow
rays, from their being next to the blue, in the order of refrangibility;
the superabundance of yellow rays, which is supplied by the candle,
gives to blue bodies, a greenish hue.
_Caroline._ Candle-light must then give to all bodies, a yellowish
tinge, from the excess of yellow rays; and yet it is a common remark,
that people of a sallow complexion, appear fairer, or whiter, by
candle-light.
_Mrs. B._ The yellow cast of their complexion is not so striking, when
every surrounding object has a yellow tinge.
_Emily._ Pray, why does the sun appear red, through a fog?
[Illustration: PLATE XXI.]
_Mrs. B._ It is supposed to be owing to the rays, which are most
refrangible, being also the most easily reflected: in passing through an
atmosphere, loaded with moisture, as in foggy weather, and also in the
morning and evening, when mists prevail, the _violet_, _indigo_, _blue_,
and _green_ rays, are reflected back by the particles which load the
air; whilst the _yellow_, _orange_, and _red_ rays, being less
susceptible of reflection, pass on, and reach the eye.
_Caroline._ And, pray, why is the sky of a blue colour?
_Mrs. B._ You should rather say, the atmosphere; for the sky is a very
vague term, the meaning of which, it would be difficult to define,
philosophically.
_Caroline._ But the colour of the atmosphere should be white, since all
the rays traverse it, in their passage to the earth.
_Mrs. B._ Do not forget that the direct rays of light which pass from
the sun to the earth, do not meet our eyes, excepting when we are
looking at that luminary, and thus intercept them; in which case, you
know, that the sun appears white. The atmosphere is a transparent
medium, through which the sun's rays pass freely to the earth; but the
particles of which it is composed, also reflect the rays of light, and
it appears that they possess the property of reflecting the blue rays,
the most copiously: the light, therefore, which is reflected back into
the atmosphere, from the surface of the earth, falls upon these
particles of air, and the blue rays are returned by reflection: this
reflection is performed in every possible direction; so that whenever we
look at the atmosphere, some of these rays fall upon our eyes; hence we
see the air of a blue colour. If the atmosphere did not reflect any
rays, though the objects, on the surface of the earth, would be
illuminated, the sky would appear perfectly black.
_Caroline._ Oh, how melancholy would that be; and how pernicious to the
sight, to be constantly viewing bright objects against a black sky. But
what is the reason that bodies often change their colour; as leaves,
which wither in autumn, or a spot of ink, which produces an iron-mould
on linen?
_Mrs. B._ It arises from some chemical change, which takes place in the
arrangement of the component parts; by which they lose their tendency to
reflect certain colours, and acquire the power of reflecting others. A
withered leaf thus no linger reflects the blue rays; it appears,
therefore, yellow, or has a slight tendency to reflect several rays,
which produce a dingy brown colour.
An ink spot on linen, at first absorbs all the rays; but, from the
action of soap, or of some other agent, it undergoes a chemical change,
and the spot partially regains its tendency to reflect colours, but with
a preference to reflect the yellow rays, and such is the colour of the
iron-mould.
_Emily._ Bodies, then, far from being of the colour which they appear to
possess, are of that colour to which they have the greatest aversion,
with which they will not incorporate, but reject, and drive from them.
_Mrs. B._ It certainly is so; though I scarcely dare venture to advance
such an opinion, whilst Caroline is contemplating her beautiful rose.
_Caroline._ My poor rose! you are not satisfied with depriving it of
colour, but even make it have an aversion to it; and I am unable to
contradict you.
_Emily._ Since dark bodies, absorb more solar rays than light ones, the
former should sooner be heated if exposed to the sun?
_Mrs. B._ And they are found, by experience, to be so. Have you never
observed a black dress, to be warmer than a white one?
_Emily._ Yes, and a white one more dazzling: the black is heated by
absorbing the rays, the white is dazzling, by reflecting them.
_Caroline._ And this was the reason that the brown paper was burnt in
the focus of the lens, whilst the white paper exhibited the most
luminous spot, but did not take fire.
_Mrs. B._ It was so. It is now full time to conclude our lesson. At our
next meeting, I shall give you a description of the eye.
Questions
1. (Pg. 179) What is meant by the refraction of light?
2. (Pg. 179) What is believed to be the cause of refraction?
3. (Pg. 180) How is a ray refracted in passing obliquely from air into
water?
4. (Pg. 180) How is this refraction explained in fig. 1, plate 19?
5. (Pg. 180) What is fig. 2 intended to explain?
6. (Pg. 180) What is the rule respecting refraction, by different
mediums?
7. (Pg. 181) What is meant by the perpendicular?
8. (Pg. 181) How does fig. 3, plate 19, elucidate the law of refraction?
9. (Pg. 181) What will be the effect on the apparent situation of the
flower?
10. (Pg. 181) What effect has refraction upon the apparent depth of a
stream of water?
11. (Pg. 182) How does the atmosphere refract the rays of the sun, as
represented, fig. 4?
12. (Pg. 182) Why have we the rays of the sun always refracted?
13. (Pg. 182) What length of time is required for light to travel from
the sun, to the earth?
14. (Pg. 182) What effect has this upon his apparent place?
15. (Pg. 182) How is the length of the day affected by refraction?
16. (Pg. 183) How are rays refracted, which fall obliquely upon a flat
pane of glass, (fig. 5, plate 19?)
17. (Pg. 183) What is the reason that objects are distorted, when seen
through common window glass?
18. (Pg. 184) What is meant by a lens?
19. (Pg. 184) What are the five kinds called, represented at fig. 1,
plate 20?
20. (Pg. 184) What is meant by the axis of a lens?
21. (Pg. 184) How are parallel rays, refracted by the double convex
lens, fig. 6, plate 19?
22. (Pg. 184) What is meant by the focus of a lens?
23. (Pg. 184) What is the focal distance of parallel rays, from a double
convex lens?
24. (Pg. 184) How are the rays refracted by a concave lens, fig. 7,
plate 19?
25. (Pg. 185) What is the effect of one plane side in a lens?
26. (Pg. 185) How is the focus of the plano-convex lens situated, fig.
2, plate 20?
27. (Pg. 185) How does a convex lens magnify objects, fig. 6, plate 19?
28. (Pg. 185) What is the article denominated which is represented at
fig. 3, plate 20?
29. (Pg. 185) How will a ray be refracted, which enters on one side of
the prism, in the direction A B?
30. (Pg. 185) What effect is produced by this refraction, as represented
in fig. 4, plate 20?
31. (Pg. 186) Of what are the rays of white light said to be composed?
32. (Pg. 186) What colours are produced?
33. (Pg. 186) By what property, in light, does refraction enable us to
separate these different rays?
34. (Pg. 187) What experiment may be performed with a piece of card, so
as to exemplify the compound nature of light?
35. (Pg. 187) How can the same be shown by a lens, fig. 5. plate 20?
36. (Pg. 187) Is it certain that there are seven primitive colours in
the spectrum?
37. (Pg. 188) How is the rainbow produced, and what is necessary to its
production?
38. (Pg. 188) How are the solar rays affected by a convex lens?
39. (Pg. 188) Why is such a lens, called a burning glass?
40. (Pg. 188) Why are bodies of a dark colour, more readily inflamed,
than those which are white?
41. (Pg. 189) What is believed to be the reason, why some bodies absorb
more rays than others?
42. (Pg. 189) What determines the colour of any particular body?
43. (Pg. 189) What exemplifications are given?
44. (Pg. 189) By what reasoning is it proved, that bodies do not retain
their colours in the dark?
45. (Pg. 190) What proof of the truth of this theory of colours, may be
afforded by the prism?
46. (Pg. 191) Why will green leaves, when exposed to the red ray, appear
of a dingy brown?
47. (Pg. 191) Bodies, in general, when placed in a ray differing in
colour from their own, appear of a mixed hue, what causes this?
48. (Pg. 191) Why will bodies of a pale, or light hue, most perfectly,
assume the different colours of the spectrum?
49. (Pg. 192) Upon what property in a body, does the darkness of its
colour depend?
50. (Pg. 192) Why do some bodies appear white, others black, and others
of different colours?
51. (Pg. 192) From what cause do blue articles appear green, by
candle-light?
52. (Pg. 193) What is believed to be the cause, of the red appearance of
the sun, through a fog, or misty atmosphere?
53. (Pg. 193) From what is the blue colour of the sky, thought to arise?
54. (Pg. 193) What would be the colour of the sky, did not the
atmosphere reflect light?
55. (Pg. 193) From what cause do some bodies change their colour, as
leaves formerly green, become brown, and ink, yellow?
56. (Pg. 194) Why is a black dress, warmer in the sunshine, than a white
one of the same texture?
CONVERSATION XVII.
ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.
DESCRIPTION OF THE EYE. OF THE IMAGE ON THE RETINA. REFRACTION BY THE
HUMOURS OF THE EYE. OF THE USE OF SPECTACLES. OF THE SINGLE MICROSCOPE.
OF THE DOUBLE MICROSCOPE. OF THE SOLAR MICROSCOPE. MAGIC LANTHORN.
REFRACTING TELESCOPE. REFLECTING TELESCOPE.
MRS. B.
The body of the eye, is of a spherical form: (fig. 1. plate 21.) it has
two membranous coats, or coverings; the external one, _a a a_, is called
the sclerotica, this is commonly known under the name of the white of
the eye; it has a projection in that part of the eye which is exposed to
view, _b b_, which is called the transparent cornea, because, when
dried, it has nearly the consistence of very fine horn, and is
sufficiently transparent for the light to obtain free passage through
it.
The second membrane which lines the cornea, and envelops the eye, is
called the choroid, _c c c_; this has an opening in front, just beneath
the cornea, which forms the pupil, or sight of the eye, _d d_, through
which the rays of light pass into the eye. The pupil is surrounded by a
coloured border called the iris, _e e_, which, by its muscular motion,
always preserves the pupil of a circular form, whether it is expanded in
the dark, or contracted by a strong light. This you will understand
better by examining fig. 2.
_Emily._ I did not know that the pupil was susceptible of varying its
dimensions.
_Mrs. B._ The construction of the eye is so admirable, that it is
capable of adapting itself, more or less, to the circumstances in which
it is placed. In a faint light, the pupil dilates so as to receive an
additional quantity of rays, and in a strong light, it contracts, in
order to prevent the intensity of the light from injuring the optic
nerve. Observe Emily's eyes, as she sits looking towards the windows:
the pupils appear very small, and the iris, large. Now, Emily, turn from
the light, and cover your eyes with your hand, so as entirely to exclude
it, for a few moments.
_Caroline._ How very much the pupils of her eyes are now enlarged, and
the iris diminished! This is, no doubt, the reason why the eyes suffer
pain, when from darkness, they suddenly come into a strong light; for
the pupil being dilated, a quantity of rays must rush in, before it has
time to contract.
_Emily._ And when we go from a strong light, into obscurity, we at first
imagine ourselves in total darkness; for a sufficient number of rays
cannot gain admittance into the contracted pupil, to enable us to
distinguish objects: but in a few minutes it dilates, and we clearly
perceive objects which were before invisible.
_Mrs. B._ It is just so. The choroid _c c_, is embued with a black
liquor, which serves to absorb all the rays that are irregularly
reflected, and to convert the body of the eye, into a more perfect
camera obscura. When the pupil is expanded to its utmost extent, it is
capable of admitting ten times the quantity of light, that it does when
most contracted. In cats, and animals which are said to see in the dark,
the power of dilatation and contraction of the pupil, is still greater;
it is computed that the pupils of their eyes may admit one hundred times
more light at one time than at another.
Within these coverings of the eye-ball, are contained, three transparent
substances, called humours. The first occupies the space immediately
behind the cornea, and is called the aqueous humour, _f f_, from its
liquidity and its resemblance to water. Beyond this, is situated the
crystalline humour, _g g_, so called from its clearness and
transparency: it has the form of a lens, and refracts the rays of light
in a greater degree of perfection, than any that have been constructed
by art: it is attached by two muscles, _m m_, to each side of the
choroid. The back part of the eye, between the crystalline humour and
the retina, is filled by the vitreous humour, _h h_, which derives its
name from a resemblance it is supposed to bear, to glass, or vitrified
substances.
[Illustration: PLATE XXII.]
The membranous coverings of the eye are intended chiefly for the
preservation of the retina, _i i_, which is by far the most important
part of the eye, as it is that which receives the impression of the
objects of sight, and conveys it to the mind. The retina is formed by
the expansion of the optic nerve, and is of a most perfect whiteness:
this nerve proceeds from the brain, enters the eye, at _n_, on the side
next the nose, and is finely spread over the interior surface of the
choroid.
The rays of light which enter the eye, by the pupil, are refracted by
the several humours in their passage through them, and unite in a focus
on the retina.
_Caroline._ I do not understand the use of these refracting humours: the
image of objects was represented in the camera obscura, without any such
assistance.
_Mrs. B._ That is true; but the representation became much more strong
and distinct, when we enlarged the opening of the camera obscura, and
received the rays into it, through a lens.
I have told you, that rays proceed from bodies in all possible
directions. We must, therefore, consider every part of an object which
sends rays to our eyes, as points from which the rays diverge, as from a
centre.
_Emily._ These divergent rays, issuing from a single point, I believe
you told us, were called a pencil of rays?
_Mrs. B._ Yes. Now, divergent rays, on entering the pupil, do not cross
each other; the pupil, however, is sufficiently large to admit a small
pencil of them; and these, if not refracted to a focus, by the humours,
would continue diverging after they had passed the pupil, would fall
dispersed upon the retina, and thus the image of a single point, would
be expanded over a large portion of the retina. The divergent rays from
every other point of the object, would be spread over a similar extent
of space, and would interfere and be confounded with the first; so that
no distinct image could be formed, and the representation on the retina
would be confused, both in figure and colour. Fig. 3. represents two
pencils of rays, issuing from two points of the tree, A B, and entering
the pupil C, refracted by the crystalline humour D, and forming on the
retina, at _a b_, distinct images of the spot they proceed from. Fig. 4.
differs from the preceding, merely from not being supplied with a lens;
in consequence of which, the pencils of rays are not refracted to a
focus, and no distinct image is formed on the retina. I have delineated
only the rays issuing from two points of an object, and distinguished
the two pencils in fig. 4. by describing one of them with dotted lines:
the interference of these two pencils of rays on the retina, will enable
you to form an idea of the confusion which would arise, from thousands
and millions of points, at the same instant pouring their divergent rays
upon the retina.
_Emily._ True; but I do not yet well understand, how the refracting
humours, remedy this imperfection.
_Mrs. B._ The refraction of these several humours, unites the whole of a
pencil of rays, proceeding from any one point of an object, to a
corresponding point on the retina, and the image is thus rendered
distinct and strong. If you conceive, in fig. 3., every point of the
tree to send forth a pencil of rays, similar to those from A B, every
part of the tree will be as accurately represented on the retina, as the
points _a b_.
_Emily._ How admirably, how wonderfully, is this contrived!
_Caroline._ But since the eye absolutely requires refracting humours, in
order to have a distinct representation formed on the retina, why is not
the same refraction equally necessary, for the images formed in the
camera obscura?
_Mrs. B._ It is; excepting the aperture through which we receive the
rays into the camera obscura, is extremely small; so that but very few
of the rays diverging from a point, gain admittance; but when we
enlarged the aperture, and furnished it with a lens, you found the
landscape more perfectly represented.
_Caroline._ I remember how obscure and confused the image was, when you
enlarged the opening, without putting in the lens.
_Mrs. B._ Such, or very similar, would be the representation on the
retina, unassisted by the refracting humours.
You will now be able to understand the nature of that imperfection of
sight, which arises from the eyes being too prominent. In such cases,
the crystalline humour, D, (fig. 5.) being extremely convex, refracts
the rays too much, and collects a pencil, proceeding from the object A
B, into a focus, F, before they reach the retina. From this focus, the
rays proceed, diverging, and consequently form a very confused image on
the retina, at _a b_. This is the defect in short-sighted people.
_Emily._ I understand it perfectly. But why is this defect remedied by
bringing the object nearer to the eye, as we find to be the case with
short-sighted people?
_Mrs. B._ The nearer you bring an object to your eye, the more divergent
the rays fall upon the crystalline humour, and consequently they are not
so soon converged to a focus: this focus, therefore, either falls upon
the retina, or at least approaches nearer to it, and the object is
proportionally distinct, as in fig. 6.
_Emily._ The nearer, then, you bring an object to a lens, the further
the image recedes behind it.
_Mrs. B._ Certainly. But short-sighted persons have another resource,
for objects which they can not bring near to their eyes; this is, to
place a concave lens, C D, (fig. 1, plate 22.) before the eye, in order
to increase the divergence of the rays. The effect of a concave lens,
is, you know, exactly the reverse of a convex one: it renders parallel
rays divergent, and those which are already divergent, still more so. By
the assistance of such glasses, therefore, the rays from a distant
object, fall on the pupil, as divergent as those from a less distant
object; and, with short-sighted people, they throw the image of a
distant object, back, as far as the retina.
_Caroline._ This is an excellent contrivance, indeed.
_Mrs. B._ And tell me, what remedy would you devise for such persons as
have a contrary defect in their sight; that is to say, who are
long-sighted, in whom the crystalline humour, being too flat, does not
refract the rays sufficiently, so that they reach the retina before they
are converged to a point?
_Caroline._ I suppose that a contrary remedy must be applied to this
defect; that is to say, a convex lens, L M, fig. 2, to make up for the
deficiency of convexity of the crystalline humour, O P. For the convex
lens would bring the rays nearer together, so that they would fall,
either less divergent, or parallel, on the crystalline humour; and, by
being sooner converged to a focus, would fall on the retina.
_Mrs. B._ Very well, Caroline. This is the reason why elderly people,
the humours of whose eyes are decayed by age, are under the necessity of
using convex spectacles. And when deprived of that resource, they hold
the object at a distance from their eyes, as in fig. 3, in order to
bring the focus more forward.
_Caroline._ I have often been surprised, when my grandfather reads
without his spectacles, to see him hold the book at a considerable
distance from his eyes. But I now understand the cause; the more distant
the object is from the crystalline lens, the nearer to it, will the
image be formed.