912 Transactions of the American Institute. 



after its passage through the prism, whether that separation be into a 

 continuous bend, as at SY, which is found when the incident light- 

 contains waves of every variety in length, or into a few separate lines, 

 as at LM, where the incident light is made up of only two sorts or 

 lengths of waves, or when, as at K, the incident light is entirely 

 made up of waves of the same length or is purely of one color, or, to 

 use a single word, is monochroniatic, and is then not separated at all, 

 but is simply refracted, and goes all to one place. 



When pure white light, such as we get from luminous solid, or, 

 indeed, dense bodies generally, is thus dispersed, separated, or, as we 

 often say, analyzed, as above, by the prism, it yields a continuous spec- 

 trum, i. e., a long band of blended colors, beginning at one end with 

 red and running through various tints of vermilion, orange, yellow, 

 green, blue and purple, to violet at the other. 



But this is not all, with the stronger sorts of light. "We find that 

 this spectrum runs on beyond the violet for a distance more than five 

 times as great as that usually visible, in what are called actinic rays ; 

 that is to say, waves so short and quick that under no ordinary con- 

 dition do they affect the sense of sight, though they are potent in 

 producing those changes- which are recognized in photography. 



For present example of this, you see on the screen a photograph 

 of the actinic or extra violet spectrum of sunlight. 



The dark lines indicated by letters we cannot pause now even to 

 allude to, otherwise than to remark that they are found all through 

 the solar spectrum, and are useful as points of reference. (See Plate 

 I, Fig. 2.) 



Slight as seems to be the difference between colors, according to- 

 the view I have just explained, *. <?., merely a question of length of 

 waves ; yet, except in the case which is the subject of our present 

 study, so fixed and unchangeable is each element of the most com- 

 posite beam, that not the least change of tint is ever found to occur. 

 Red light, though it be passed through a hundred prisms or lenses or 

 other optical machines, cannot be made one atom more or less red, 

 and still less, therefore, changed to yellow or green or any other 

 color. 



But, you may well ask, to what then are due all the colors which 

 we see in nature ? Does not the white light find itself changed into 

 green when it falls on a leaf, and into red when it encounters a brick 

 wall ? 



I answer no. White light consists of or contains all colors. When 

 it falls upon a leaf all the colors but such as produce to us the effect 



