SIX LECTURES ON LIGHT, 



then we produce the water-spectrum ; finally, 

 by introducing a flint glass prism, we refract 

 the beam back, until the color cisappears. 

 The image of the slit is now white ; but you 

 see that, though the dispersion is abolished, 

 the refraction is not. 



This is the place to illustrate another point 

 bearing upon the instrumental means em- 

 ployed in these lectures. Note the position 

 ot the water-spectrum upon the screen. Alter- 

 ing, in no particular, the wedge-shaped ves- 

 sel, but simply substituting for the water the 

 transparent bisulphide of carbon, you notice 

 how much higher the beam is thrown, and 

 how much richer is the display of color. 

 This will explain to you the use of this sub- 

 stance in our subsequent experiments. 



The synthesis of white light may be 

 effected in three ways, which are now worthy 

 of special attention: Here, in the first in- 

 stance, we have a rich spectrum produced by 

 a prism of bisulphide of carbon. One face 

 of the prism is protected by a diaphragm 

 with a longitudinal slit, through which the 

 beam passes into the prism. It emerges de- 

 composed at the other side. I permit the 

 colors to pass through a cylindrical lens, 

 which so squeezes them together as to pro- 

 duce upon the screen a sharply-defined rect- 

 angular image of the longitudinal slit. In 

 that image the colors are re-blended, and you 

 see it perfectly white. Between the prism 

 and the cylindrical lens may be seen the 

 colors tracking themselves through the dust 

 Df the room. Cutting off the more refrangi- 

 ble fringe by u card, the rectangle is seen 

 red ; cutting off the less refrangible fringe, 

 the rectangle is seen blue. By means of a 

 thin glass prism, I deflect one portion of the 

 colors, and leave the residual portion. On 

 the screen are now two colored rectangles 

 produced in this way. These are comple- 

 mentary colors colors which, by their union, 

 produce white. Note that, by judicious 

 management, one of these colors is icndered 

 yellow, and the other blue. I withdraw the 

 thin prism ; yellow falls upon blue, and we 

 have white as the result of their union. On 

 our way, we thus abolish the fallacy first 

 exposed by Helmholtz, that the mixture of 

 blue and yellow lights produces green. 



Again, restoring the circular aperture, we 

 obtain once more a spectrum like that of 

 Newton. By means of a lens, we gather up 

 these colors, and build them together not to 

 an image of the aperture, but to an image of 

 the carbon points themselves. Finally, in 

 virtue of the persistence of impressions upon 

 the retina, by means of a rotating disk, on 

 which are spread in sectors the colors of the 

 spectrum, we blend together the prismatic 

 colors in the eye itself, and thus produce the 

 impression of whiteness. 



Having unravelled the interwoven con- 

 stituents of white light, we have next to 

 inquire, What part the constitution s:> 

 revealed enables this agent to play in Nature ? 



To it we owe all the phenomena of color; 

 and yet not to it alone, for there must be a 

 certain relationship between the ultimate par- 

 ticles of natural bodies and light to enable 

 them to extract from it the luxuries of color. 

 But the function of natural bodies is here 

 selective, not creative. There is no color gen- 

 erated by any natural body whatever Natural 

 bodies have showered upon them, in the 

 white light of the sun, the sum total of all 

 possible colors, and their action is limited to 

 the sifting of that total, the appropriating 

 from it of the colors which really belong to 

 them, and the rejecting of those which do 

 not. It will fix this subject in your minds if 

 I say that it is the portion of light which 

 they reject, and not that which belongs to 

 them, that gives bodies their colors. 



Let us begin our experimental inquiries here 

 by asking, \Vhatisthemeaningof blackness? 

 Pass a black ribbon in succession through the 

 colors of the spectium ; it quenches all. 

 This is the meaning of blackness it is the 

 result of the absorption of all the constituents 

 of solar light. Pass a red r'.bbon through the^ 

 spectrum. In the red light the ribbon is a 

 vivid red. "Why ? Because the light that 

 enters the ribbon is not quenched or absorbed, 

 but sent back to the eye. Place the same rib. 

 bon in the green or blue of the spectrum ; \fe 

 is black as jet. It. absorbs the green ai/4. 

 blue light, and leaves the s pace on which they 

 fall a space of intense darkness. Place a. 

 green ribbon in the green of the spectrum. 

 It shines vividly with its proper color ; transfei;- 

 it to the red, it is black as jet. Here it ab- 

 sorbs all the light that falls upon it, and offers , 

 mere darkness to the eye. Whert white light 

 is employed, the red sifts it by quenching the 

 green, and the green sifts it by quenching the 

 red, both exhibiting the residual color. Thus 

 the process through which natural bodies ac- 

 quire their colors is a negative one. The 

 colors are produced by subtraction, not by 

 addition. This red glass is , red because it 

 destroys all the more refrangible rays of the 

 spectrum. This blue liquid is blu,e^ because it, 

 destroys all the less refrangible rays. Both 

 together are opaque because the light trans- 

 mitted by the one is quenched by the other. 

 In this way by the union of two transparent, 

 substances we obtain a combination as dark 

 as pitch to solar li^ht. This other liquid, 

 finally is purple because it destroys the gree.i 

 and the yellow, and allows the terminal colors . 

 of the Lpectrum to pass unimpeded. From 

 thp blending of the blue and the red this gor- 

 geous color is produced. 



These experiments prepare us for the fur-, 

 ther consideration of a point already adverted 

 to, and regarding which error has found cur- 

 rency for ages. You will find it stated in 

 books that blue and yellow lights mixed to- 

 gether produce green. But blue and yellow 

 nave been just proved to be complementary 

 colors, producing white by their mixture. 

 The mixture of blue and yellow pigments un- 



