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SECOND DIVISION.

SECTION VIII.-PROPERTIES OF LIGHT.-OPTICS.

CHAPTER XXXIV.

PROPERTIES OF LIGHT.

Theories of the Nature of Light-Sources of Light-Phosphorescence— Temperature of a red Heat-Effects of Bodies on Light-Passage in straight Lines-Production of Shadows-Umbra and Penumbra.

HAVING treated of the mechanical properties of gases, liquids, solids, and the laws of motion, we are now led to the consideration of certain agents or forces-light, heat, electricity. These, by many philosophers, are believed to be matter in an imponderable state; they are therefore spoken of as imponderable substances. By others their effects are regarded as arising from motions or modifications impressed on a medium everywhere present, which passes under the name of THE ETHER.

Applying these views to the case of light, two different hypotheses respecting its constitution obtain. The first, which has the designation of the theory of emission, regards light as consisting of particles of amazing minuteness, which are projected by the shining body in all directions, and in straight lines. These, impinging eventually on the organ of vision, give rise to the sensation which we speak of as brightness or light. To the other theory the title of undulatory theory is given; it supposes that there exists throughout the universe an ethereal medium, in which vibratory movements can arise somewhat analogous to the movements which give birth to sounds in the air; and these passing through the transparent parts of the eye, and falling on the retina, affect it with their pulsations, as waves in the air affect the auditory nerve, but in this case give rise to the sensation of light, as in the other to sound.

There are many different sources of light-some astronomical and some terrestrial. Among the former may be mentioned the sun and the starsamong the latter, the burning of bodies, or combustion, to which we chiefly resort for our artificial lights, as lamps, candles, gas flames. Many bodies are phosphorescent; that is to say, emit light after they have been exposed to the sun or any shining source. Thus oyster-shells, which have been calcined with sulphur, shine in a dark place after they have been exposed to the light, and certain diamonds do the same. So, too, during processes of putrefaction, or slow decay, light is very often emitted, as when wood is mouldering, or meat is becoming putrescent. The source of the luminousness, in these cases, seems to be the same as in ordinary combustions; that

is, the burning away of carbon and hydrogen under the influence of atmospheric air; but, in certain cases, the functions of life give rise to an abundant emission of light, as in fireflies and glowworms: these continue to shine even under the surface of water, and there is reason to believe that the phenomenon, to a considerable extent, is subject to the volition of the animal. All solid substances, when they are exposed to a certain degree of heat, become incandescent, or emit light. When first visible in a dark place, this light is of a reddish colour; but as the temperature is carried higher and higher it becomes more brilliant, being next of a yellow, and lastly of a dazzling whiteness. For this reason, we sometimes indicate the temperature of such bodies, in a rough way, by reference to the colour they emit: thus we speak of a red heat, a yellow heat, a white heat. I have recently proved that all solid substances begin to emit light at the same degree of heat, and that this answers to 977° of Fahrenheit's thermometer; moreover, as the temperature rises, the brilliancy of the light rapidly increases, so that at a temperature of 2600° it is almost forty times as intense as at 1900°. these high temperatures an elevation of a few degrees makes a prodigious difference in the brilliancy. Gases require to be brought to a far higher temperature than solids before they begin to emit light.

At

Non-luminous bodies become visible by reflecting the light which falls on them. In their general relations, such bodies may be spoken of as transparent and opaque. By the former we mean those which, like glass, afford a more or less ready passage to the light through them; by the latter such as refuse it a passage. But transparency and opacity are never absolute— they are only relative. The purest glass extinguishes a certain amount of the rays which fall on it, and the metals which are commonly looked upon as being perfectly opaque allow light to pass through them, provided they are thin enough. Thus gold leaf spread upon glass transmits a greenishcoloured light.

The rays of light, from whatever source they may come, move forward in straight lines, continuing their course until they are diverted from it by the interposition of some obstacle, or the agency of some force. That this rectilinear path is. followed may be proved by a variety of facts. Thus, if we intervene an opaque body between any object and the eye, the moment the edge of that body comes to the line which connects the object and the eye, the object is cut off from our view. In a room into which a sunbeam is admitted through a crevice, the path which the light takes, as is marked out by the motes that float in the air, is a straight line.

By a ray of light we mean a straight line drawn from the luminous body, marking out the path along which the shining particles pass.

A shining body is said to radiate its light, because it projects its luminous particles in straight lines, like radii, in every direction, and these falling on opaque bodies, and being intercepted by them, give rise to the production of shadows.

If the light is emitted by a single luminous point, the boundary of the shadow can be obtained by drawing straight lines from the luminous point to every point on the edge of the body, and producing them. Thus, let a, Fig. 208, be the luminous point, be the opaque body; by drawing the lines, a b, a c, and producing them to d and e, the boundary and figure of the shadow may be exhibited.

But if the luminous body, as in most instances is the case, possesses a

sensible magnitude; if it is, for example, the sun or a flame, an opaque body will cast two shadows, which pass respectively under the names of the umbra and penumbra-the former being dark, and the latter partially

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illuminated. This may be illus-
trated by Fig. 209, in which a b is
the flame of a candle, or any other
luminous source, having a sensible
magnitude, c d the opaque body.
Now the straight lines, a cf, a dh,
drawn from the top of the flame to
the edges of the opaque body and
produced, give the shadow for that
point of the
flame; and
the lines bce,
b dg, drawn
in like man-

ner from the bottom of the flame, give the shadow for that point. But we see that the space between g and h, which belongs to the shadow for the top of the .. flame, is not perfectly dark, because it is so situated as to be partially illuminated by the bottom of the flame-and a similar remark may be made as respects the space, fe, which receives light from the top of the flame. But the remaining space, fg, receives no light whatever it is totally dark-and we therefore call it the umbra, while the partially illuminated regions, fe and g h, are the penumbra.

Fig. 209.

CHAPTER XXXV.

OF THE MEASURES OF THE INTENSITY AND VELOCITY OF LIGHT.

Conditions of the Intensity of Light-Of Photometric Methods-Rumford's Method by Shadows-Ritchie's Photometer-Difficulties in Coloured Lights -Masson's Method-Velocity of Light determined by the Eclipse of Jupiter's Satellites-The same by the Aberration of the Fixed Stars.

By Photometry* we mean the measurement of the brilliancy of light-an operation which can be conducted in many different ways.

It is to be understood that the illuminating power of a shining body depends on several circumstances. First, upon its distance-for near at hand the effect is much greater than far off-the law for the intensity of light in this respect being, that the brilliancy of the light is inversely as the square of the distance. A candle two feet off gives only one-fourth of the light that it does at one foot; at three feet it gives only one-ninth, &c. Secondly, it depends on the absolute intensity of the luminous surface: thus we have seen that a solid, at different degrees of heat, emits very different amounts of light; and in the same way the flame of burning hy

* This term is derived from the two Greek words, phose (pwc), light, and metron μετρον), a measure.

drogen is almost invisible, and that of spirits of wine is very dull when compared with an ordinary lamp. Thirdly, it depends on the area or surface the shining body exposes, the brightness being greater according as that surface is greater. Fourthly, in the absorption which the light suffers in passing the medium through which it has to traverse-for even the most transparent obstructs it to a certain extent. And lastly, on the angle at which the rays strike the surface they illuminate, being most effective when they fall perpendicularly, and less in proportion as their obliquity increases. The first and last of the conditions here mentioned, as controlling the intensity of light—the effect of distance and of obliquity-may be illustrated as follows:

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Fig. 210.

B

1st. That the intensity of light is inversely as the squares of the distance. Let B, Fig. 210, be an aperture in a piece of paper, through which rays coming from a small illuminated point, A, pass; let these rays be received on a second piece of paper, C, placed twice as far from A as is B, it will be found that they illuminate a surface which is twice as long and twice as broad as A, and therefore contains four times the area. If the paper be placed at D, three times as far from A as is B, the illuminated space will be three times as long and three times as broad as A, and contain nine times the surface. If it be at E, which is four times the distance, the surface will be sixteen times as great. All this arises from the rectilinear paths which the diverging rays take, and therefore a surface illuminated by a given light will receive, at distances represented by the numbers 1, 2, 3, 4, &c., quantities of light represented by the numbers 1,,, ,&c., which latter are the inverse squares of the former numbers. 2nd. That the intensity of light is dependent on the angle at which the rays strike the receiving surface, being most effective when they fall perpendicularly, and less in proportion as the obliquity increases.

Fig. 211.

Let there be two surfaces, D C and E C, Fig. 211, on which a beam of light, A B, falls on the former perpendicularly, and on the latter obliquely the latter surface, in proportion to its obliquity, must have a larger area to receive all the rays which fall on D C. A given quantity of light,

therefore, is diffused over a greater surface when it is received obliquely, and its effect is correspondingly less.

To compare different lights with one another, Count Rumford invented a process which goes under the name of the method of shadows. The principle is very simple. Of two lights, that which is the most brilliant will cast the deepest shadow, and with any light the shadow which is cast becomes less dark as the light is more distant. If, therefore, we wish to examine experimentally the brilliancy of two lights on Rumford's method, we take a screen of white paper, and setting in front of it an opaque rod, we place the lights in such a position that the two shadows arising shall be close together, side by side. Now the eye can, without any difficulty, determine which of the two is darkest; and by removing the light, which has cast it to a greater distance, we can, by a few trials, bring the two shadows to precisely the same degree of depth. Now measure the distances of the two lights from the screen, and the illuminating powers are as the squares of those distances.

Ritchie's photometer is an instrument for obtaining the same result; not, however, by the contrast of

shadows, but by the equal illu-
mination of surfaces. It con-
sists of a box, a b, Fig. 212, six
or eight inches long, and one
broad and deep, in the middle of
which a wedge of wood, feg, with
its angle, e, upward, is placed.
This wedge is covered over with
clean white paper, neatly doubled
to a sharp line at e.
of the box there is a conical tube,

In the top

Fig. 212.

with an aperture, d, at its upper end, to which the eye is applied, and the whole may be raised to any suitable height by means of the stand, c. On looking down through d, having previously placed the two lights, m n, the intensity of which we desire to determine on opposite sides of the box, they illuminate the paper surfaces exposed to them, e f to m, and e g to n, and the eye, at d, sees both those surfaces at once. By changing the position of the lights, we eventually make them illuminate the surfaces equally, and then measuring their distances from e, their illuminating powers are as the squares of those distances.

It is not possible to apply either of these methods in a satisfactory manner, where, as is unfortunately often the case, the lights to be examined differ in colour. The eye can form no judgment whatever of the relation of brightness of two surfaces when they are of different colours; and a very slight amount of tint completely destroys the accuracy of these processes. To some extent, in Ritchie's instrument, this may be avoided by placing a coloured glass at the aperture, d.

A third photometric method has recently been introduced; it has great advantages over either of the foregoing; and difference of colour, which in them is so serious an obstacle, serves in it actually to increase the accuracy of the result. The principle on which it is founded is as follows: If we take two lights, and cause one of them to throw the shadow of an opaque body upon a white screen, there is a certain distance to which, if we bring

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