SIX LECTURES ON LIGHT. 



doubtedly produces green, but the mixture of 

 pigments is totally different from the mixture 

 of lights. Helmholtz, who first proved yel- 

 low and blue to complementary ..colors, has 

 revealed the cause of the green in the case of 

 the pigments. No natural color is pure. A 

 blue liquid or a blue powder permits not only 

 the blue to pass through it, but a portion of 

 the adjacent green. A yellow powder is 

 transparent not only to the yellow light, but 

 also in part transparent to the adjacent green. 

 Now, when blue and yellow are mixed to- 

 gether, the blue cuts off the yellow, the orange, 

 and the red ; the yellow, on the other hand, 

 cuts off the violet, the indigo, and the blue. 

 Green is the only color to which both are 

 transparent, and the consequence is that, 

 when white light falls upon a mixture of yel- 

 low and blue powders, the green alone is sent 

 back to the eye. I have already shown you 

 that the fine blue ammonia-sulphate of copper 

 transmits a large portion of green, while cut- 

 ting off all the less refrangible light. A yel- 

 low solution of picric acid also allows the 

 green to pass, but quenches all the more re- 

 frangible light. What must occur when we 

 send a beam through both liquids ? The 

 green band of the spectrum alone remains 

 upon the screen. 



This question of absorption is one of the 

 most subtle and difficult in molecular physics. 

 Vvc are not yet in a condition to grapple with 

 : but we shall be by-and-by. Meanwhile, 

 "'we may profitably glance back on the web of 

 jwtlations which these experiments reveal to 

 its. We have, in the first place, in solar 

 light an agent of exceeding complexity, com- 

 posed of innumerable constituents, refrangi- 

 ble in different degrees. We find, secondly, 

 tike atoms and molecules of bodies gifted 

 writ la the power of sifting solar light in the 

 most various ways, and producing by this 

 ifting the colors observed in nature and art. 

 To do this they must possess a molecular j 

 'Structure commensurate in complexity with j 

 that of light itself. Thirdly, we have :he j 

 .human eye and brain so organized as to be 

 able to take in and distinguish the multitude 

 Oif impressions thus generated. Thus, the 

 light .at starting is complex; to sift and select 

 ic as they do natural bodies must be complex. 

 Fiaally, to take in the impressions thus gen- 

 vcjated, the human eye and brain must be 

 iiighly complex. Whence this triple com 

 p.txiy? If what are called material pur- 

 pios^s were the only end to be served, a much 

 simpler mechanism would be sufficient. But, 

 instead of simplicity instead of the princi- 

 ple of parsimony we have prodigality of re- 

 lation and adaptation, and this apparently 

 for the sole purpose of enabling us to see 

 things robed in the splendor of color. Would 

 it not seem that Nature harbored the inten- 

 tion <<if educating us for other enjoyments 

 than those derivable from meat and drink ? 

 At all events, whatever Nature meant and 

 k would, be. mere ..pres. mption to dogmatize 



as to what she meant we find ourselves here 

 as the issue and upshot of her operations, en- 

 dowed with capacities to enjov not only the 

 materially useful, but endowed with others of 

 indefinite scope and application, which deal 

 alone with the beautiful and the true. 



LECTURE II. 



Origin of Physical Theories : Scope of the Imagina- 

 tion : Newton and the Emission Theory: Verifica- 

 tion of Physical Theories: The Luminiferous ; 

 Ether: Wave-Theory of Light: Thomas Young: 

 Fresnel and Arago : Conceptions of Wave-Motion: 

 Interference of Wavts; Constitution of Sound- 

 Waves: Analogies of Sound and Light: Illustra- 

 tions of Wave-Motion: Interference of Sound- 

 Waves : Optical Illustrations: Pitch and Color: 

 Lengths of the Waves of Light and Kates of 

 Vibration of the Ether- Particles: Interference of 

 Light : Phenomena which first suggested the Un- 

 dulatory Theory: Hooke and the .Colors of Thin 

 Plates: The Soip-Bubble: Newton's Rings: 

 Theory of lv Fits: " Its Explanation of the Kings : 

 Overthrow of the Tneory : Colors of Mother-uf- 

 Pearl. 



WE might vary and extend our experi- 

 ments on light indefinitely, and they cer- 

 tainly would prove us to possess a wonderful 

 mastery over the phenomena. But the vts- 

 ture of the agent only would thus be re- 

 vealed, not the agent itself. The human 

 mind, however, is so constituted and so edu- 

 cated as regards natural things, that it can 

 never rest satisfied with this outward view of 

 them. Brightness and freshness take pos- 

 session of the mind when it is crossed by the 

 light of principles, which show the facts of 

 Nature to be organically connected. 



Let us, then, inquire what this thing is 

 that we have been generating, reflecting, re- 

 fracting, and analyzing. 



In doing this, we shall learn that the life 

 of the experimental philosopher is twolold. 

 He lives, in his vocation, a life of the senses, 

 using his hands, eyes, and ears in his experi- 

 ments, but such a question as that now before 

 us carries him beyond the margin of the 

 senses. He cannot consider, much less an- 

 swer, the question, "What is light?" with- 

 out transporting himself to a world which 

 undelies the sensible one, and out of wnich, 

 in accordance with rigid law, all optical phe- 

 nomena spring. To realize this subsensible 

 world, if 1 may use the term, the mit..d must 

 possess a certain pictorial power. It has to 

 visualize the invisible. It must be able to 

 form definite images of the things which that 

 subsensib'e world contains ; and to say that, 

 if such or such a state of things exist in that 

 world, then the phenomena which appear in 

 ours must, of necessity, grow out of this 

 state of things. If the picture be correct, 

 the phenomena are accounted for ; a physical 

 theory has been enunciated which unites and 

 explains them all. 



This conception of physical theory implies, 

 as you perceive, the exercise of the imagina- 

 tion. Do not be afraid of this word, which 

 seems to rentier so many respectable people, 



