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plan. These correspond in size to the fixed apertures, m n, but are situated at
angular distances of 120° from each other: so that, whenever light is entering
by the outer aperture m, in the fixed lid, the inner aperture, n, is closed; and
whenever the object of which the phosphorescence is to be tested is exposed to
the observer, no light from without can reach it. Now, if the object be visible
to the observer at o, whilst the disk is in rotation, it can only become so by a
phosphorescent action; and, by varying the rate of rotation, the interval between
the action of the light on the sensitive surface, and the exposure of the object to
the eye of the observer, can be made to vary from a period as short as the
of a second to any greater interval. Other and still more sensitive forms of the
instrument have been employed; but for a description of these the reader is
referred to the original memoirs above cited.

The colour of the light emitted by these phosphori is peculiar to each substance, and seldom corresponds with that of the incident ray: it is generally of a lower degree of refrangibility, never of higher refrangibility; for instance, baric sulphide emits a yellow light, though excited by the violet and extra-violet rays: and calcic sulphide, which in different specimens emits an orange, a green, or a blue phosphorescence, is in all cases excited by the more refrangible portion of the spectrum beyond the line G.

The cause of the variation in tint of the phosphorescence produced by different specimens of the same substance, has been minutely examined by Becquerel; and he attributes it to molecular, and not to chemical differences in the phosphori, the results being influenced by the temperature at which the phosphorescent body was prepared, and the crystalline structure and greater or less compactness of the material (e.g., calcic sulphate or carbonate) employed in the preparation of the phosphori. A phosphorescent body, which has been fused, and allowed to solidify again, when placed in the phosphoroscope, often emits light of a tint different from that which it exhibited before it had undergone fusion; for example, plates of crystallized boracic acid furnish a greenish-blue light, but after the acid has been fused the phosphorescence is yellow. Loaf-sugar emits a pale greenish light, but after fusion, on again exposing it in the phosphoroscope, it gives off a much more intense yellowish light.

It is to be remarked that, in many cases, the less refrangible rays of the spectrum actually destroy the phosphorescence produced by the more refrangible rays. Where the phosphorescence has a considerable duration, it is found that elevation of temperature heightens the luminosity, but shortens the duration of the phosphorescence. The effect of heat upon strontic sulphide, when prepared with due precaution, is very remarkable. Certain specimens of it at o° F. (— 18° C.) emit a very beautiful violet phosphorescence; by raising the temperature to 158° (70° C.), the light emitted has a greenish hue, and if the tube which contains the sulphide be heated to about 392° (200° C.) the light becomes of an orange yellow.

Becquerel is of opinion that the phenomena of phosphorescence and those of fluorescence have a common origin-many phosphorescent bodies, such as uranic nitrate, æsculin, and quinine sulphate, emitting light of the same tint as that which they display when fluorescent. This point, however, requires further investigation, since many bodies which are highly phosphorescent show no signs of fluorescence, and the range of colour in the light emitted by phosphorescent bodies is smaller than in the same bodies when they become fluorescent.

(113) Velocity of Light.-It is certain that light is the result of a series of progressive actions, since it requires time for its propagation. This was first noticed by Roemer, in 1675 and 1676, who ascertained from observations on the eclipses or

FIG. 90.

occultations of the satelites of Jupiter, that when the earth, as re

presented at F, fig. 90, is situated at its greatest distance from that pla

net, I, these occulta

tions appear to occur about a quarter of an hour later than they do when the earth is nearest to it, as at E; consequently, between 15 and 16 minutes are required by light in traversing the width, E F, of the earth's orbit, a distance of about 190,000,000 miles.* It has been found by careful observations that the greatest apparent retardation of the eclipses is 16 min. 26'6 sec., or 986'6 sec.: taking the diameter of the earth's orbit as 183,000,000 miles, the velocity of light will be 185,481 miles (298,505,300 metres) per second.

183,000,000
986'6

=

In 1849 some experiments were made by Fizeau (Comptes Rendus, 1849, xxix. 90), demonstrating that the velocity of light could be determined on the surface of the earth through a distance of only 5'364 miles (8633 metres). For these experiments a telescope was fitted at Suresnes, having at the focus of the object-glass a toothed wheel, the width of the teeth, and of the intervals between them, being equal. When the wheel was rotated an object viewed through the eyepiece was alternately eclipsed and rendered visible for equal spaces of time. By a suitable arrangement a beam of light could be passed along the telescope, issuing from the object-glass in parallel rays. At Montmartre (8633 metres from Suresnes) the light was received on the object-glass of another telescope, the eyepiece of which was replaced by a plane mirror, which reflected the light along its original path, and into the object-glass of the first telescope where it was observed. When the opaque tooth of the wheel was at the focus of the telescope, of course no light issued from the object-glass; but when the tooth was replaced by an interval,

Or, according to more recent computations of astronomers, 182,000,000 miles (23,573 radii of the earth; Delaunay). When the results of the observations of the transit of Venus on Dec. 9, 1874, have been calculated, an accurate determination of this distance will be obtained.

114.] VELOCITY OF LIGHT-FREQUENCY OF VIBRATION.

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the light was observed, after traversing a distance of 17,266 metres (10728 miles). On causing the wheel to rotate rapidly the observer noticed that the light was gradually diminished, and ultimately eclipsed. This happened when the time occupied by the light in travelling from Suresnes to Montmartre and back, was equal to the time necessary for the replacement of the interval between two teeth by the following tooth. On doubling the velocity of rotation of the wheel the light again became visible, the light now passing through one space, and returning through the next. By determining the rate of the rotation of the wheel the velocity of the light was calculated, and found to be 313,274,200 metres (194,663 miles) per second.

These experiments have been repeated by Cornu (Comptes Rendus, 1874, lxxix. 1361) with more perfect apparatus, who has found the velocity to be 300,400,000 metres (186,663 miles) a second.

Foucault, by a beautiful application of the revolving mirror, first used by Wheatstone, determined the velocity through the short interval of 20 metres (65618 feet) to be at the rate of 298,187,000 metres (185,287 miles) a second. The diameter of the earth's orbit may, in fact, be calculated from these terrestrial experiments by multiplying the velocity by the time occupied in the passage of the light across the orbit. This being 986 6 seconds, gives by Cornu's results 186,663 × 986·6=184,162,000 miles. Light would therefore traverse a distance equal to the circumference of the earth in about the eighth part of a second of time.

The velocity of light, however, varies with the medium through which it passes; in a more refracting medium its velocity is diminished; but in a medium of uniform density it travels in a uniform direction, and its velocity is also uniform. It may be shown mathematically that if the hypothesis of emission be correct, the velocity must be increased in a more refracting medium, whilst on the undulatory theory it should be diminished; the decision of this question, therefore, affords an experimentum crucis between the two theories.

Foucault (Ann. Chim. Phys. 1854 [3], xli. 129) has succeeded in solving this important question; by direct measurement he found that light is retarded in the more powerfully refracting medium: the relative velocity being inversely as the refractive indices of the media compared. Consequently the theory of emission cannot be longer maintained.

(114) Length and Frequency of Undulations of Light.-The

undulatory hypothesis accounts for differences in the intensity of the ray of light R s, fig. 91, by differences in the amplitude or

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FIG. 91.

excursion, a b, of the undulation; and for the phenomena of colour by differences in the length, a c, and in s the frequency of the undulations; just as in the phenomena of sound, the pitch of the note is proved to depend upon the number of waves or impulses which occur in a given time: if one note be an octave higher than another note, it will have twice the number of vibrations in the same interval of time; while the length of each wave will be just half that of the waves which produce the lower note: but the extent through which the ear appreciates proportionate differences of rapidity in the undulations which produce sound, is much greater than that which the eye can estimate in the case of light. Most persons can perceive musical sounds in which all possible variety exists between 16 and 2048 vibrations in a second, i.e., including a range of seven octaves, in the highest of which the vibrations are 128 times more numerous than in the lowest. With light the range is much more limited, and extends not quite so far as from 1 to 2.* The average length of a wave of white light is of an inch, or omm.000508, which is nearly equal to the thickness of 6 leaves of gold; but the length of the wave, as well as its frequency, differs in the different colours: in red light it is longer, being about o.in.000029937, or 45 of an inch, or omm.0007604, while in violet it is only o.in.000015484, or 5 of an inch or omm.0003933 according to the measurements of Angström. The number of vibrations is estimated at five hundred and eighty-six million millions per second (586,c cooco,000000, or 586 × 1012) in white light; in red light at 392,ccc000,00coco, or 392 × 10"; and in violet light at as much as 764,000000,000000, or 764 × 10".

The length of the waves of light may be varied by motion of the luminous body or the observer towards or away from one another. This may be well illustrated by reference to the corresponding phenomenon observed in the case of sound. This subject was very beautifully described by Professor Stokes in his inaugural address to the British Association at Exeter in 1869The pitch of a musical note depends, as we know, on the number of vibrations which reach the ear in a given time, such as a second.

The range in the invisible portion of the spectrum is, however, much greater for the rays which produce fluorescence and chemical action.

114.]

FREQUENCY OF VIBRATION.

Suppose, now, that a body, such as a given number of times per second, is from the observer, the air being calm.

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bell, which is vibrating a at the same time moving Since the successive pulses

of sound travel all with the velocity of sound, but diverge from different centres, namely, the successive points in the bell's path at which the bell was when those pulses were first excited, it is evident that the sound-waves will be somewhat more spread out on the side from which the bell is moving, and more crowded together on the side towards which it is moving, than if the bell had been at rest. Consequently the number of vibrations per second which reach the ear of an observer situated in the former of these directions will be somewhat smaller, and the number which reach an observer situated in the opposite direction somewhat greater, than if the bell had been at rest. Hence to the former the pitch will be somewhat lower, and to the latter somewhat higher, than the natural pitch of the bell. And the same thing will happen if the observer be in motion instead of the bell, or if both be in motion; in fact, the effect depends only on the relative motion of the observer and the bell in the direction of a line joining the two,-in other words, on the velocity of recession or approach of the observer and the bell. The effect may be perceived in standing by a railway when a train in which the steam-whistle is sounding passes by at full speed, or better still, if the observer be seated in a train which is simultaneously moving in the opposite direction.'

Dr. Pole (Nature, 1875, xi. 232) has calculated the drop of the note of a railway whistle. as observed when an engine passes an observer in another train travelling in the opposite direction. As the trains approach, the note is raised, but, after passing, it will be lowered: the drop is the difference between the two

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A similar phenomenon is often strikingly exhibited when the observer is travelling rapidly past the engine of a heavy goods train; on approaching the

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