SIX LECTURES ON LIGHT. 



along m r E, there is also refraction at E, the 

 beam striking the point n f . From the ends 

 of the incident beams, let the perpendiculars 

 111 o, m' o / ba drawn upon 1) D, and from the 

 ends of the refr-cted beams let the perpen- 

 diculars p n,p' n f be also drawn. Measure 

 the lengths of o in and of p n. and divide the 



one by the ether. You obtain a certain quo- 

 tient. In like manner divide m' o f by the 

 corresponding perpendicular p r ;/; you ob- 

 tain in each case the same quotient. Snell, in 

 fact, found this quotient to be a constant 

 quantity for each particular substance, though 

 ic varied in amount from substance to sub- 

 st.mce He called the quotient the index of 

 refraction. 



This law is one of the corner-stones of 

 optical science, and its applications to-day 

 are million-fold. Immediately after its dis- 

 covery, Descartes applied it to the explana- 

 tion of the rainbow. The bow :s seen when 

 tbe back is turned to the sun. Draw a 

 straight line through the spectator's eye and 

 the sun, the bow is always seen at the same 

 angular distance from this line. This was 

 the great difficulty. Why should the bow be 

 always and at all its parts, forty-one degrees 

 from this line ? Taking a pen and calculat- 



certain that he did net enunciate the true 

 law. This was reserved for Newton, who 

 went to work in this way: Through the closed 

 window-shutter of a room he pieiced an ori- 

 fice, and allowed a thin sunbeam to pass 

 through it. The beam stamped a round 

 image of the sun on the opposite white wall 

 of the room. In the path of this beam New- 

 ton placed a prism, expecting to see the beam 

 refracted, but also expecting to see the image 

 of the sun, af'cr refi action, round; to his 

 astonishment, it was drawn out to an image 

 whose length was five times its breadth; and 

 this image was divided into bands of differ- 

 ent colors. Newton saw immediately that 

 solar light was composite, not simple. His 

 image revealed to him the fact that some con- 

 stituents of the solar light were more deflect- 

 ed by the prism than others, and he conclud- 

 ed, therefore, that white solar light was a 

 mixture of lights of different colors and of 

 different degrees of rcfrangibility. 



Let us reproduce this celebrated experi- 

 ment. On tne screen is now stamped a lu- 

 minous disk, which may stand for Newton's 

 image of the sun. Causing the beam wl ich 

 produces the disk to pass through a pri-rn, 

 we obtain Newton's elongated colored image, 

 which he called a spectrum. Newton divided 

 the spectrum into seven parts red, orange, 

 yellow, green, blue, indigo, violet which 

 are commonly called the seven primary or 

 prismatic colors. This drawing out of the 

 white light into its constituent colors is called 

 dispersion. 



This was the first analysis of solar light bv 

 Newton ; but the scientific mind is fond of 

 verification, and never neglects it where it is 

 possible. It is this stern conscientiousness in 

 testing its conclusions that gives adamantine 

 trength to science, and renders all assaults 

 on it unavailing. Newton completed his 

 proof by synthesis in this way : The spec- 

 rum now before you is produced by a glass 

 prism. Causing the decomposed beam to 



ing the track of every ray through a rain- pass through a second simitar prism, but so 

 drop, Descartes found that, at one particular placed that the colors are refracted back and 



angle, the rays emerged from the drop almost 

 parallel to each other; being t :us enabled to 

 preserve their intensity through long atmos- 

 pheric distances; at all other angles the rays 

 quitted the drop divergent, and through this 

 divergence became so enfeebled as to be 

 practically ;ost to the eye. The particular 

 angle here referred to was the ioregoing 

 angle of forty-one degrees, which observa- 

 tion had proved to be invariably that of the 



rainbow. 

 But in 



the rainbow a new phenomenon 



was introduced the phenomenon of color. 

 And here we arrive at one of those points in 

 the history of science, when men's labors so 

 intermingle, that it is difficult to assign to 

 each worker his precise meed cf honor. Des- 

 cartes was at the threshold of the discovery 

 of the composition of solar light. But he 

 failed to attain perfect clearness, and it is 



reblended, the perfectly white image of the 

 slit is restored. Here, then, refraction and 

 dispersion are simultaneously abolished. Are 

 they always so ? Can we have the one with- 

 out the other? It was Newton's conclusion 

 that we could not. Here he erred, and his 

 error, which he maintained to the end of his 

 life, retarded the progress of optical discovery. 

 Dolland subsequently proved that, by com 

 bining two different kinds of glass, the color 

 could be extinguished, still leaving a rcsidui 

 of refraction, and he employed this residue 

 in the construction of achromatic lenses - 

 lenses which yield no color which Newion 

 thought an impossibility. By setting a water 

 prism water contained in a wedge-shaped 

 vessel with glass sides in opposition to a 

 prism of glass, this point can be illustrated 

 before you. We have first the position of the 

 unrefracted beam marked upon the screen ; 



