Scientific Lectures. 159 



He found tliat upon passing a beam of white light through a glass 

 prism, it became divided into differently colored rays. If a beam of 

 light, of sunlight for example, enters a dark room through a slit in 

 the shutter, that beam of light, as every one knows, crosses the room 

 in a rectilinear direction, and forms on the opposite wall an image of 

 the slit. The image is very indefipite, it is true ; but if we interpose 

 between the slit and the wall a glass lens, in a proper position, we 

 shall obtain a perfectly defined image of the slit. If now we inter- 

 pose in the path of the same beam a prism, we find that that prism 

 produces two distinct results. In the first place, it bends the beam 

 of light, so that the image of the slit, instead of being upon the oppo- 

 site wall, is thrown very much to the left ; and, in the second place, 

 it spreads out that beam just like the rays of a fan ; and instead of a 

 single image of the slit, it forms a broad band of blending colors, such 

 as is represented here, and which we call the solar spectrum. If now, 

 by any means, we recombine these different colored rays, we obtain, as 

 I shall show yon at the close of the lecture, a pure white light again. 

 From all this, it follows that white light is a very complex sensation. 

 It is simply the confused impression produced upon the eye by the 

 simultaneous effect of light of every different shade of color. Pure 

 color, on the other hand, is a simple sensation, and corresponds pre- 

 cisely to a pure musical note. But most natural colors are not pure 

 colors. For example, the color which the ladies call purple is a 

 complex color, formed by the blending of various tints of red and 

 violet. We can easily discover the different hues of which any given 

 color consists by simply passing the light that emanates from the 

 color through a glass prism, which will divide up the complex color 

 into its different simple tints. On account of the great interest which 

 attaches to studies of this kind, an instrument has been invented for 

 analyzing colors, to which we give the name of " spectroscope." "We 

 have one of these instruments before us. It is not necessary for me 

 to enter into any detailed description of its mechanism. In fact, the 

 mechanism is extremely simple. It consists of the several parts I 

 have already pointed out. In the first place we have here the slit 

 through which the light enters. We have in the interior a glass 

 prism which bends the light, so that the light entering from the slit 

 passes in the direction of this brass tube, the second tube making, as 

 you see, a decided angle with the first. Moreover, we have a small 

 telescope, the whole object of which is to give a definite image af the 

 slit. If we direct this instrument toward any source which gives us 



