THE SUN — ITS CHEMICAL ANALYSIS. 177 



is produced whenever the luminous rays pass from one substance into 

 another, explains all that play of light which we witness in the water, 

 in the atmosphere, in all transparent mediums. We owe to Newton 

 the first scientific explanation of it. 



Let us suppose ourselves in a chamber entirely darkened, into which 

 light enters only by an extremely narrow slit made in a shutter. Let 

 a glass prism be presented in the line which the entering sheet of 

 rays is traversing, and a leaf of paper be placed at a distance of some 

 feet beyond and opposite the prism. The different rays which com- 

 pose the white light are not refracted in the same degi'ee in passing 

 from the air into the glass, nor again in quitting the glass to traverse 

 the air anew: consequently, in place of a luminous white line, we 

 shall see on the paper a rectangle covered with differently colored 

 bands. Newton distinguished therein seven principal colors: violet, 

 indigo, blue, green, yellow, orange, and red; in reality, the tints 

 pass into one another by an insensible and harmonious transition. 

 To this expansion of the luminous beam has been given the name of 

 solar spectrum, though with little justice, for nothing spectral can 

 attach itself in idea to that beautiful sheet of light, whose tones the 

 richest pallet would fail to reproduce. The colored bands of the 

 rainbow are merely a solar spectrum, pale and much weakened, pro- 

 duced by the refraction of the rays in the minute drops of rain; the 

 play of light which is so fine in soap bubbles better represents the 

 brilliancy of the spectrum in the darkened chamber. 



So long as curiosity was limited to the simple effects produced by 

 the interposition of the prism and the reception of the refracted rays 

 on a sheet of paper or other white surface, there was nothing seen in 

 the spectrum but the seven elementary colors, without further dis- 

 coveries; but the spectrum, like every luminous object, may be studied 

 with magnifying optical instruments, and it was by exploring it in 

 this way that, about 1814, the German savant Frauenhofer remarked 

 some singular properties, whose discovery Avill immortalize his name. 

 The spectrum, as we have seen, is formed of an infinite number of 

 bands of different tints united together; the flag, as it were, of nature, 

 not tricolore, but omnicolore; and among these colored parallel zones 

 Franenhofer was the first to perceive bands or rather lines of black, 

 not only towards the two extremities of the spectrum, where the light 

 is merged in the surrounding obscurity, but in the most brilliant parts 

 and in all the colors. He recognized these lines as having the same 

 invariable place in the spectrum, and since that time they have pre- 

 served the alphabetic name which he assigned them: we still say the 

 line A, B, or C, of Frauenhofer, and in thus speaking physicists know 

 at once in what part of the spectrum these rays are found. 



With more delicate instruments and more perfect prisms, there 

 have been observed in the spectrum many more dark lines than Frau- 

 enhofer had indicated. In 1860, the English philosopher Sir David 

 Brewster, to whom optics owes so many happy discoveries, presented 

 a drawing of the spectrum crossed by a multitude of these lines, and 

 M. Kirchoff has employed, in the course of the observations just com- 

 pleted, so delicate an apparatus that, to use his own expression, 

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