COLORIFIC ABSORPTION. 101 



389. A piece of sheet lead, about a quarter of an inch thick, is to be cut into the 

 form of a horse-shoe, of such magnitude that a circle one inch in diameter might be 

 inscribed in it. Upon this lead two pieces of very pure and transparent crown glass 

 are cemented, so as to form a trough, for containing a variety of liquids. It is well to 

 accommodate this trough with a strong foot, or basis, a a, and several such troughs may 

 be provided. Fig. 40, c c c, the leaden horse-shoe ; b b, the glass plates. 



390. A thin metallic plate, three or four inches square, having a longitudinal slit 

 about one inch long and T V inch wide in it, is also to be provided. It is convenient 

 that this, too, should be furnished with a pediment Fig. 41, a a, the slit. 



391. The lens (g, fig. 39) having been removed, by turning the screws a beam of 

 light is to be thrown horizontally into the room; the screen (Jig. 41) is then to be placed 

 before the brass tube,yi so that the slit in it may allow a narrow streak of light to pass. 

 The trough (fig. 40) is then placed behind, in such a position that half the light which 

 comes through the slit in the screen may pass through the liquid contained in the 

 trough, and the other half pass by its side unintercepted. This arrangement is shown 

 in fig. 42. Behind the trough is placed a flint glass prism, d,fig. 43, and, farther still, 

 a white pasteboard screen, e, of suitable dimensions, a being the metallic screen, b the 

 trough. 



392. The action of this arrangement is as follows : the beam of light cast by the 

 mirror into the room is entirely intercepted, except the small portion which passed 

 through the slit in the metallic screen. A part of this passes through the trough, and 

 a part on one side of it, the middle part being obstructed by the leaden horseshoe. 

 Two beams of light, therefore, fall on the prism, one of which has passed through the 

 trough, aud one which has not, and they are separated from each other by a dark in- 

 terval. The prism disperses both, and there fall on the pasteboard screen two spectra, 

 side by side, close enough for a very accurate examination. One of them has been 

 acted on by the fluid in the trough, the other is undisturbed. In my arrangement, the 

 spectrum a happens to be the natural one, and b the disturbed one (fig. 44). 



393. Let us now take an example, as an illustration of the use of this apparatus. 

 Fill the trough with distilled water, and let the mirror throw a horizontal beam. 

 Two spectra are seen on the screen, e,fig. 43, close to each other, side by side, with a 

 dark interval between them. They contain, as may be perceived, all the seven col- 

 ours of Newton, nor does the one differ in any wise from the other, as in fig. 45. 



394. Having poured the water out of the trough, fill it with a strong, but clear solu- 

 tion of the chromate of potassa ; on looking at the spectra on the screen, a is still found 

 of its natural appearance, but by the side of it there is a distorted spectrum, formed by 

 the light that has passed through the trough ; the blue, the indigo, and the violet rays 

 are wanting, as is seen in fig. 46. These colours have then been absorbed by the so- 

 lution of chromate of potassa. If this solution be poured out, and one of sulphate of 

 copper and ammonia poured into the trough, another kind of spectrum is produced, 

 w here the red and much of the yellow light is wanting, as in fig. 47. If a strong solu- 

 tion of Brazil wood is used, the disturbed spectrum will be found to have lost its violet, 

 indigo, blue, green, yellow, and a great part of its orange rays, as represented in fig. 48. 



