Let a beam of sun-light be admitted through a small opening into a dark room. If allowed to fall normally on a white screen, it produces (§ 938) a round white spot, which is an image of the sun. Now let a prism be placed in its path edge-downwards, as in Fig. 755; the

beam will thus be deflected upwards, and at the same time resolved into its component colours. The image depicted on the screen will be a many-coloured band, resembling the spectrum of white described in § 1045. It will be of uniform width, and rounded off at the ends, being in fact built up of a number of overlapping discs, one for each kind of elementary ray. It is called the solar spectrum.

The rays which have undergone the greatest deviation are the violet. They occupy the upper end of the spectrum in the figure. Those which have undergone the least deviation are the red. Of all visible rays, the violet are the most, and the red the least refrangible; and the analysis of light into its components by means of the prism is due to difference of refrangibility. If a small opening is made in the screen, so as to allow rays of only one colour to pass, it will be found,

on transmitting these through a second prism behind the screen, as in Fig. 755, that no further analysis can be effected, and the whole of the image formed by receiving this transmitted light on a second screen will be of this one colour.

1048. Mode of obtaining a Pure Spectrum.-The spectra obtained by the methods above described are built up of a number of overlapping images of different colours. To prevent this overlapping, and obtain each elementary colour pure from all admixture with the rest, we must in the first place employ as the object for yielding the images a very narrow line; and in the second place we must take care that the images which we obtain of this line are not blurred, but have the greatest possible sharpness. A spectrum possessing these characteristics is called pure.

The simplest mode of obtaining a pure spectrum consists in looking through a prism at a fine slit in the shutter of a dark room. The edges of the prism must be parallel to the slit, and its distance from the slit should be five feet or upwards. The observer, placing his eye close to the prism, will see a spectrum; and he should rotate the prism on its axis until he has brought this spectrum to its smallest angular distance from the real slit, of which it is the image.

Let E (Fig. 756) be the position of the eye, S that of the slit. Then the extreme red and violet images of the slit will be seen at R, V, at distances from the prism sensibly equal to the real distance of S (§ 997); and the other images, which compose the remainder

of the spectrum, will occupy positions between R and V. The spectrum, in this mode of operating, is virtual.

To obtain a real spectrum in a state of purity, a convex lens must be employed. Let the lens L (Fig. 757) be first placed in such a position as to throw a sharp image of the slit S upon

Fig. 756. —— Arrangement for seeing a Pure Spectrum. a screen at I. Next let a prism P be introduced between the lens

and screen, and rotated on its axis till the position of minimum deviation is obtained, as shown by the movements of the impure spectrum which travels about the walls of the room. Then if the screen be moved into the position R V, its distance from the prism being the

PURE SPECTRUM.

1063 same as before, a pure spectrum will be depicted upon it. A similar result can be obtained by placing the prism between the lens and the slit, but the adjustments are rather more troublesome. Direct

Fig. 757.-Arrangement for Pure Spectrum on Screen.

sun-light, or sun-light reflected from a mirror placed outside the shutter, is necessary for this experiment, as sky-light is not sufficiently powerful. It is usual, in experiments of this kind, to employ a movable mirror called a heliostat, by means of which the light can be reflected in any required direction. Sometimes the movements of the mirror are obtained by hand; sometimes by an ingenious clock-work arrangement, which causes the reflected beam to keep its direction unchanged notwithstanding the progress of the sun through the heavens.

The advantage of placing the prism in the position of minimum deviation is twofold. First, the adjustments are facilitated by the equality of conjugate focal distances, which subsists in this case and in this only. Secondly and chiefly, this is the only position in which the images are not blurred. In any other position it can be shown1 that a small cone of homogeneous incident rays is no longer a cone (that is, its rays do not accurately pass through one point) after transmission through the prism.

The method of observation just described was employed by Wollaston, in the earliest observations of a pure spectrum ever obtained. Fraunhofer, a few years later, independently devised the same method, and carried it to much greater perfection. Instead of looking at the virtual image with the naked eye, he viewed it through a telescope, which greatly magnified it, and revealed several features never before detected. The prism and telescope were at a distance of 24 feet from the slit.

1 Parkinson's Optics, § 96. Cor. 2.

1049. Dark Lines in the Solar Spectrum.-When a pure spectrum of solar light is examined by any of these methods, it is seen to be traversed by numerous dark lines, constituting, if we may so say, dark images of the slit. Each of these is an indication that a particular kind of elementary ray is wanting1 in solar light. Every elementary ray that is present gives its own image of the slit in its own peculiar colour; and these images are arranged in strict contiguity, so as to form a continuous band of light passing by perfectly gradual transitions through the whole range of simple colour, except at the narrow intervals occupied by the dark lines. Fig. 1, Plate III., is a rough representation of the appearance thus presented. If the slit is illuminated by a gas flame, or by any ordinary lamp, instead of by solar light, no such lines are seen, but a perfectly continuous spectrum is obtained. The dark lines are therefore not characteristic of light in general, but only of solar light.

Wollaston saw and described some of the more conspicuous of them. Fraunhofer counted about 600, and marked the places of 354 upon a map of the spectrum, distinguishing some of the more conspicuous by the names of letters of the alphabet, as indicated in fig. 1. These lines are constantly referred to as reference marks for the accurate specification of different portions of the spectrum. They always occur in precisely the same places as regards colour, but do not retain exactly the same relative distances one from another, when prisms of different materials are employed, different parts of the spectrum being unequally expanded by different refracting substances.2 The inequality, however, is not so great as to introduce any difficulty in the identification of the lines.

The dark lines in the solar spectrum are often called Fraunhofer's lines. Fraunhofer himself called them the "fixed lines."

1050. Invisible Rays of the Spectrum. The brightness of the solar spectrum, however obtained, is by no means equal throughout, but is greatest between the dark lines D and E; that is to say, in the yellow and the neighbouring colours orange and light green; and falls off gradually on both sides.

The heating effect upon a small thermometer or thermopile increases in going from the violet to the red, and still continues to increase for a certain distance beyond the visible spectrum at the red end. Prisms and lenses of rock-salt should be employed for this

'Probably not absolutely wanting, but so feeble as to appear black by contrast. 2 This property is called the irrationality of dispersion.

PHOSPHORESCENCE.

1065 investigation, as glass largely absorbs the invisible rays which lie beyond the red.

When the spectrum is thrown upon the sensitized paper employed in photography, the action is very feeble in the red, strong in the blue and violet, and is sensible to a great distance beyond the violet end. When proper precautions are taken to insure a very pure spectrum, the photograph reveals the existence of dark lines, like those of Fraunhofer, in the invisible ultra-violet portion of the spectrum. The strongest of these have been named L, M, N, O, P.

1051. Phosphorescence and Fluorescence.-There are some substances which, after being exposed in the sun, are found for a long time to appear self-luminous when viewed in the dark, and this

without any signs of combustion or sensible elevation of temperature. Such substances are called phosphorescent. Sulphuret of calcium and sulphuret of barium have long been noted for this property, and have hence been called respectively Canton's phosphorus, and Bologna