1. The word optician is applicable to persons who are particularly
skilled in the science of vision, but especially to those who devote
their attention to the manufacture of optical instruments, such as the
spectacles, the camera obscura, the magic lantern, the telescope, the
microscope, and the quadrant.
2. Light is an emanation from the sun and other luminous bodies, and
is that substance which renders opaque bodies visible to the eye. It
diverges in a direct line, unless interrupted by some obstacle, and
its motion has been estimated at _two hundred thousand miles_ in a
second.
3. A _ray of light_ is the motion of a single particle: and a parcel
of rays passing from a single point, is called _a pencil of rays_.
_Parallel rays_ are such as always move at the same distance from each
other. Rays which continually approach each other, are said to
_converge_; and when they continually recede from each other, they are
said to _diverge_. The point at which converging rays meet is called
the _focus_.
4. Any pellucid or transparent body, as air, water, and glass, which
admits the free passage of light, is called a _medium_. When rays,
after having passed through one medium, are bent out of their original
course by entering another of different density, they are said to be
_refracted_; and when they strike against a surface, and are sent back
from it, they are said to be _reflected_.
5. A _lens_ is glass ground in such a form as to collect or disperse
the rays of light which pass through it. These are of different
shapes; and they have, therefore, received different appellations. A
_plano-convex_ lens has one side flat, and the other convex; a
_plano-concave_ lens is flat on one side, and concave on the other; a
_double convex_ lens is convex on both sides; a _double concave_ lens
is concave on both sides; a _meniscus_ is convex on one side, and
concave on the other. By the following cut, the lenses are exhibited
in the order in which they have been mentioned.
[Illustration]
6. An _incident ray_ is that which comes from any luminous body to a
reflecting surface; and that which is sent back from a reflecting
surface, is called a _reflected ray_. The _angle of incidence_ is the
angle which is formed by the incident ray with a perpendicular to the
reflecting surface; and the _angle of reflection_ is the angle formed
by the same perpendicular and the reflected ray.
7. When the light proceeding from every point of an object placed
before a lens is collected in corresponding points behind it, a
perfect image of the object is there produced. The following cut is
given by way of illustration.
[Illustration]
8. The lens, _a_, may be supposed to be placed in the hole of a
window-shutter of a darkened room, and the arrow at the right to be
standing at some distance without. All the light reflected from the
latter object towards the lens, passes through it, and concentrates,
within the room, in a focal point, at which, if a sheet of paper, or
any other plane of a similar color, is placed, the image of the object
will be seen upon it.
9. This phenomenon is called the _camera obscura_, or dark chamber,
because it is necessary to darken the room to exhibit it. The image at
the focal point within the room is in an inverted position. The reason
why it is thrown in this manner will be readily understood by
observing the direction of the reflected rays, as they pass from the
object through the lens. In the camera obscura, it is customary to
place a small mirror immediately behind the lens, so as to throw all
the light which enters, downwards upon a whitened table, where the
picture may be conveniently contemplated.
10. From the preceding explanation of the camera obscura, the theory
of vision may be readily comprehended, since the eye itself is a
perfect instrument of this kind. A careful examination of the
following representation of the eye will render the similarity
obvious. The eye is supposed to be cut through the middle, from above
downwards.
[Illustration: _a a_, the _sclerotica_; _b b_, the _choroides_; _c c_,
the _retina_; _d d_, the _cornea_; _e_, the _pupil_; _f f_, the
_iris_; _g_, the _aqueous humor_; _h_, the _crystalline humor_; _i i_,
the _vitreous humor_.]
11. The _sclerotica_ is a membranous coat, to which the muscles are
attached which move the eye. The _cornea_ is united to the sclerotica
around the circular opening of the latter, and is that convex part of
the eye, which projects in advance of the rest of the organ. The space
between this and the crystalline lens is occupied by the aqueous humor
and the iris. The _iris_ is united to the choroides, and it possesses
the power of expanding and contracting, to admit a greater or less
number of rays.
12. The _crystalline lens_ is a small body of a crystalline appearance
and lenticular shape, whence its name. It is situated between the
aqueous and vitreous humors, and consists of a membranous sack filled
with a humor of a crystalline appearance. The _vitreous humor_ has
been thus denominated on account of its resemblance to glass in a
state of fusion. The _retina_ is a membrane which lines the whole
cavity of the eye, and is formed chiefly, if not entirely, by the
expansion of the optic nerve.
13. The rays of light which proceed from objects pass through the
cornea, aqueous humor, crystalline lens, and vitreous humor, and fall
upon the retina in a focal point, to which it is brought, chiefly by
the influence of the cornea and the crystalline lens. The image, in
an inverted position, is painted or thrown on the cornea, which
perceives its presence, and conveys an impression of it to the brain,
by means of the optic nerve.
14. _Optical instruments._--The art of constructing optical
instruments is founded upon the anatomical structure, and
physiological action of the eye, and on the laws of light. They are
designed to increase the powers of the eye, or to remedy some defect
in its structure. In the cursory view which we may give of a few of
the many optical instruments which have been invented, we will begin
with the _spectacles_, since they are the best known, and withal the
most simple.
15. The _visual point_, or the distance at which small objects can be
distinctly seen, varies in different individuals. As an average, it may
be assumed at eight or nine inches from the eye. In some persons, it is
much nearer, and in others, considerably more distant. The extreme, in
the former case, constitutes _myopy_, or _short-sightedness_, and, in
the latter case, _presbyopy_, or _long-sightedness_.
16. _Myopy_ is chiefly caused by too great a convexity of the cornea
and the crystalline lens, which causes the rays to converge to a
focus, before they reach the retina. Objects are, therefore,
indistinctly seen by myoptic persons, unless held very near the eye to
throw the focus farther back. This defect may be palliated by the use
of concave glasses, which render the rays proceeding from objects more
divergent.
17. _Presbyopy_ is principally caused by too little convexity of the
cornea and crystalline lens, which throws the focal point of rays
reflected from near objects, beyond the retina. This defect is
experienced by most people, to a greater or less degree, after they
have advanced beyond the fortieth year, and occasionally even by
youth. A remedy, or, at least, a palliation, is found in the use of
convex glasses, which render the rays more convergent, and enable the
eye to refract them to a focus farther forward, at the proper point.
18. The opticians have their spectacles numbered, to suit different
periods of life; but, as the short-sighted and long-sighted conditions
exist in a thousand different degrees, each person should select for
himself such as will enable him to read without effort at the usual
distance.
19. The great obstacle to viewing small objects at the usual distance,
arises from too great a divergence of the light reflected from them,
which causes the rays to reach the retina before they have converged
to a focus. This defect is remedied by convex lenses, which bring the
visual point nearer to the eye, and consequently cause the rays to
concentrate in a large focus upon the retina. The most powerful
microscopic lenses are small globules of glass, which permit the eye
to be brought very near to the object.
20. _Microscopes_ are either _single_ or _double_. In the former case,
but one lens is used, and through this the object is viewed directly;
but, in the latter case, two or more glasses are employed, through one
of which a magnified image is thrown upon a reflecting surface, and
this is viewed through the other glass, or glasses, as the real object
is seen through a single microscope.
21. The _solar microscope_, on account of its great magnifying powers,
is the most wonderful instrument of this kind. The principles of its
construction are the same with those of the camera obscura. The
difference consists chiefly in the minor circumstance of placing the
object very near the lens, by which a magnified image is thrown at the
focal point within the room.
22. In the case of the camera obscura, the objects are at a far
greater distance from the glass on the outside than the images, at the
focal point, on the inside. The comparatively great distance of the
object, in this case, causes the image to be proportionably smaller.
In the solar microscope, a small mirror is used to receive the rays,
and to reflect them directly upon the object.
23. The _magic lantern_ is an instrument used for magnifying paintings
on glass, and for throwing their images upon a white surface in a
darkened room. Its general construction is the same with that of the
solar microscope; but, in the application, the light of a lamp is
employed instead of that from the sun.
24. _Telescopes_ are employed for viewing objects which from their
distances appear small, or are invisible to the naked eye. They are of
two kinds, _refracting_ and _reflecting_. The former kind is a
compound of the camera obscura and the single microscope. It consists
of a tube, having at the further end a double convex lens, which
concentrates the rays at a focal point within, where the image is
viewed through a microscopic lens, placed at the other end.
25. In the construction of reflecting telescopes, concave mirrors, or
specula, are combined with a double convex lens. A large mirror of
this kind is so placed in the tube, that it receives the rays of light
from objects, and reflects them upon another of a smaller size. From
this they are thrown to a focal point, where the image is viewed
through a double convex lens. The specula are made of speculum metal,
which is a composition of certain proportions of copper and tin.
26. Many optical appearances are of such frequent recurrence, that
they could not have escaped the earliest observers; nevertheless, ages
appear to have elapsed, before any progress was made towards an
explanation of them. Empedocles, a Greek philosopher, born at
Agrigentum in Sicily, 460 years before Christ, is the first person on
record who attempted to write systematically on light.
27. The subject was successively treated by several other
philosophers; but the ancients never attained to a high degree of
information upon it. We have reason to believe, however, that convex
lenses were, in some cases, used as magnifiers, and as burning
glasses, although the theory of their refractive power was not
understood.
28. The magnifying power of glasses, and some other optical phenomena,
were largely treated by Al Hazen, an Arabian philosopher, who
flourished about the year 1100 of our era; and, in 1270, Vitellio, a
Polander, published a treatise on optics, containing all that was
valuable in Al Hazen's work, digested in a better manner, and with
more lucid explanations of various phenomena.
29. Roger Bacon, an English monk, who was born in 1214, and who lived
to the age of seventy-eight, described very accurately the effects of
convex and concave lenses, and demonstrated, by actual experiment,
that a small segment of a glass globe would greatly assist the sight
of old persons. Concerning the actual inventor of spectacles, however,
we have no certain information; we only know that these useful
instruments were generally known in Europe, about the beginning of the
fourteenth century.
30. In the year 1575, Maurolicus, a teacher of mathematics, at
Messina, published a treatise on optics, in which he demonstrated that
the crystalline humor of the eye is a lens, which collects the rays of
light from external objects, and throws them upon the retina. Having
arrived at a knowledge of these facts, he was enabled to assign the
reasons why some people were short-sighted, and others long-sighted.
31. John Baptista Porta, of Naples, was contemporary with Maurolicus.
He invented the camera obscura, and his experiments with this
instrument convinced him, that light was a substance, and that its
reception into the eye produced vision. These discoveries corresponded
very nearly with those by Maurolicus, although neither of these
philosophers had any knowledge of what the other had done. The
importance of Porta's discoveries will be evident, when it is
observed, that, before his time, vision was supposed to be dependent
on what were termed _visual rays_, proceeding from the eye.
32. The telescope was invented towards the latter end of the sixteenth
century. Of this, as of many other valuable inventions, accident
furnished the first hint. It is said, that the children of Zacharias
Jansen, a spectacle-maker, of Middleburg in Holland, while playing
with spectacle-glasses in their father's shop, perceived that, when
the glasses were held at a certain distance from each other, the dial
of the clock appeared greatly magnified, but in an inverted position.
33. This incident suggested to their father the idea of adjusting two
of these glasses on a board, so as to move them at pleasure. Two such
glasses inclosed in a tube completed the invention of the simplest
kind of the refracting telescope. Galileo greatly improved the
telescope, and constructed one that magnified thirty-three times, and
with this he made the astronomical discoveries which have immortalized
his name.
34. John Kepler, a great mathematician and astronomer, who was born at
Weir, in Wurtemburg, in the year 1571, paid great attention to the
phenomena of light and vision. He was the first who demonstrated that
the degree of refraction suffered by light in passing through lenses,
corresponds with the diameter of the circle of which the concavity or
convexity is the portion of an arch. He very successfully pursued the
discoveries of Maurolicus and Porta, and asserted that the images of
external objects were formed upon the optic nerve by the concentration
of rays which proceed from them.
35. In 1625, the curious discovery of Scheiner was published, at Rome,
which placed beyond doubt the fact, that vision depends upon the
formation of the image of objects upon the retina. The fact was
demonstrated by cutting away, at the back part, the two outside coats
of the eye of an animal, and by presenting different objects before
it. The images were distinctly seen painted on the naked retina.
36. Near the middle of the seventeenth century, the velocity of light
was discovered by Roemer; and, in 1663, James Gregory, a celebrated
Scotch mathematician, published the first proposal for a reflecting
telescope. But, as he possessed no mechanical dexterity himself, and
as he could find no workman capable of executing his designs, he never
succeeded in carrying his conceptions into effect. This was reserved
for Sir Isaac Newton; who, being remarkable for manual skill, executed
two instruments of this kind, in the year 1672, on a plan, however,
somewhat different from that proposed by Gregory.
37. In the course of the year 1666, the attention of Sir Isaac Newton
was drawn to the phenomena of the refraction of light through the
prism; and, having observed a certain surprising fact, he instituted a
variety of experiments, by which he was brought to the conclusion,
that light was not a homogeneous substance, but that it is composed of
particles, which are capable of different degrees of refrangibility.
38. By the same experiments, he also proved, that the rays or
particles of light differ from each other in exhibiting different
colors, some producing the color red, others that of yellow, blue, &c.
He applied his principles to the explanation of most of the phenomena
of nature, where light and color are concerned; and almost every thing
which we know upon these subjects, was laid open by his experiments.
39. The splendor of Sir Isaac Newton's discoveries obscures, in some
measure, the merits of earlier and subsequent philosophers; yet
several interesting discoveries in regard to light and color, as well
as many important improvements of optical instruments, have been made
since his time, although the light by which these have been achieved,
was derived principally from his labors.
[Illustration: GOLDBEATER.]