UNIV. OF

or oblong aperture, a very narrow slit is employed, light of certain degrees of refrangibility is absent from the solar beam. He admitted the light through a narrow slit (parallel to the refracting angle of the prism). With this arrangement the spectrum seen by Wollaston was not continuous, but crossed by two dark lines parallel to the slit-or, in other words, at right angles to the length of the spectrum.

These two lines-two gaps in the solar spectrum— proved that light of two definite orders of refrangibility is absent from the solar beam. Fig. 22 shows how

FIG. 22.

the light, after passing through the prism, had become
divided into three parts, with spaces between them
along which no light travelled. It is quite obvious that
the existence of these gaps can be recognised without
allowing a spectrum to be formed on a screen, as in
Newton's experiment-simply by placing the eye as
shown in fig. 22. It was in this
It was in this way that Wollaston
observed the two gaps. It may be remarked in passing
that this mode of viewing the spectrum bears the same
relation to Newton's plan that observation of the Sun
with the naked eye bears to observation of the Sun's
image received upon a screen.

Dr. Wollaston did not pursue the inquiry further.
Nor need we greatly wonder at this, if we rightly con-

germ

sider the matter. We now know, indeed, that in the two dark spaces on the spectrum of Wollaston there lay the of the most wonderful discoveries man has yet made. made. We know that had he persisted in the inquiry his name would have been associated through all time, as that of Fraunhofer will undoubtedly be, with the very language of the new analysis. But it must be admitted that Wollaston had little reason for expecting any very remarkable results from the study of a peculiarity which seemed quite as likely to depend on the nature of the glass of which the prism was made as upon any inherent property of solar light. And even supposing that the gaps were due to some peculiarity of solar light, who could suspect that that peculiarity, when traced to its source, would be so full of meaning as to reveal to us the very constitution of the solar orb?

It is to the unwearying patience with which Fraun hofer-like so many others of his countrymen—was willing to work day after day at what seemed a most unpromising subject, that the world owes the first complete recognition of the characteristic peculiarities of the solar spectrum. Wollaston had observed the spectrum directly, with the unaided eye. Fraunhofer improved on this plan by employing a telescope.*

*It is well to notice that all the modes of viewing the Sun are available for viewing the solar spectrum. A prismatic spectrum is simply a series of images of a luminous object formed by rays of different refrangibilities. Where the luminous object is a line of light-as where light is received through a fine slit-the solar spectrum is in reality made up of an infinite number of lines of light at right angles to its length. It is because light of certain definite orders of refrangibility is

Dr. Wollaston had seen but two gaps in the solar spectrum; Fraunhofer, in 1814, saw and mapped no less than 576 lines. The positions of the chief lines, seen by Fraunhofer are indicated in fig. 23, and as reference is continually made to the lettered lines, it is well that the student should carefully study their sequence and position.

A is a well-marked line close to the limits of visibility at the red end of the spectrum. B is a welldefined red line of sensible breadth.*

B is a band of several lines called a.

Between A and

c is a dark and

very well-marked line. Between B and c Fraunhofer counted nine fine lines; between C and D about thirty. D consists of two strong lines close together. Between D and E Fraunhofer counted eighty-four lines. E is a band of several lines, the middle line of the set being stronger than the rest. At b are three strong lines,

wanting that images of the line are wanting at certain definite parts of the spectrum, that, in other words, dark lines are seen. The clear recognition of this fact will prevent much misapprehension.

It will be understood that I am here describing the lines as seen by Fraunhofer, and as they would appear with similar spectroscopic power to that employed by him. With greater power many single lines are resolved into several, and many new lines make their appearance. For example, D is described above as consisting of two strong lines with an extremely small interval. With a powerful spectroscope numerous lines are seen between these two strong lines.

the two farthest from E being close together. Between E and Fraunhofer counted twenty-four lines, and between b and F more than fifty. F, G, and H are strong lines. Between F and G, and between G and H, Fraunhofer counted 185 and 190 lines respectively; and he found many lines also between H and 1-the violet end of the spectrum.

Fig. 23 shows the colours of those parts of the spectrum in which the several lines occur. The reader will do well to bear in mind the position of the several lines, as thus, by an easily remembered relation, he will find himself enabled to interpret readily the accounts of spectroscopic researches, whether into astronomical or chemical subjects of inquiry. Let him remember, then, that A, B, and c are in the red portion of the spectrum; D in the orange-yellow, E in the yellowish-green; F in the greenish-blue; G in the indigo; and H in the violet.

Fraunhofer's contributions to the science of spectroscopic analysis did not conclude, however, with the recognition and mapping of these lines. Having first convinced himself that the same lines were seen in the solar spectrum, of whatever substance the prism was formed, he proceeded to study the spectra formed by light from other sources.

He first examined solar light received indirectly by reflection or otherwise-as from the clouds, the sky, the Moon or planets, and so on; and he found in the spectrum of such light the same lines which he had seen in the spectrum of direct solar light. He

studied the spectrum of the Sun when that orb is near the horizon, and he found that under such circumstances the violet end of the spectrum disappears and a number of lines make their appearance in the remainder of the spectrum.

Fraunhofer found that the spectra of the fixed stars exhibit dark lines resembling those in the Sun; but none of the stars whose light he examined had a spectrum exactly the same as the Sun's. Some lines of the solar spectrum he found wanting in star spectra, while other lines were not seen with the same relative distinctness as in the spectrum of the Sun. On the other hand, he found several new lines in star spectra. No two stars appeared to have the same spectrum.

An important conclusion followed, as Fraunhofer pointed out, from this observation. If the dark lines in the solar spectrum were caused by an absorptive action exercised by our own atmosphere, it would follow that the same lines ought to be seen in the spectra of the fixed stars. The contrary being the case, Fraunhofer held it to be a demonstrated fact, that the dark lines. are due to some property inherent in the light itself which the Sun and the fixed stars severally emit.

One more observation of Fraunhofer's, and I pass on to later researches. He found that when the flame of a candle or lamp is the source of light, the spectrum is not crossed by dark lines, but a bright double line is seen in the exact place occupied by the double dark line D of the solar spectrum. To prevent misconception, it is necessary, however, to mention that light from an