SIX LECTURES ON LIGHT. 



that the function of natural bodies is selec- 

 tive, not creative ; that they extinguish cer- 

 tain constituents of the \vhite solar light, and 

 appear in the colors of the unextinguished 

 iight. It must atonce flash upon your minds 

 that, inasmuch as we have in interference an 

 agency by which light may be self-extin- 

 quished, we may have in it the conditions 

 for the production of color. But this would 

 imply that certain constituents are quenched 

 by interference, while others are permitted to 

 remain. This is the fact ; and it is entirely 

 due to the difference in the lengths of the 

 waves of light. 



The subject is most easily illustrated by the 

 class of phenomena which (irst suggested the 

 undulatory theory to the mind of Hooke. 

 These are the colors of thin films of all kinds, 

 which are known as the colors of thin plates. 

 In this relation no object in the world pos- 

 sesses a deeper scientific interest than a com- 

 mon soap-bubble. And here let me say 

 emerges one of the difficulties which the stu- 

 dent of pure science encounters in the pres- 

 ence of "practical" communities like those 

 of America and England ; it is not to be ex- 

 pected that such communities can entertain 

 any profound sympathy with labors which 

 seem so far removed from the domain of 

 practice as many of the labors of the man of 

 science are. Imagine Dr. Draper spending 

 his days in blowing soap-bubbles and in 

 studying their colors ! Would you show him 

 the necessary patience, or grant him the nec- 

 essary support ? And yet, be it remembered, 

 it was thus that Newton spe::t a large portion 

 of his time ; and that on such experiments 

 has been founded a theory, the issues of 

 which are incalculable. I see no other way 

 for you laymen than to trust the scientific man 

 with the choice of his inquiries ; he stands 

 before the tribunal of his peers, and by their 

 verdict on his labors you ought to abide. 



Whence, then, are derived the colors of the 

 soap-bubble ? Imagine abeam of white l : ght 

 impinging on the bubble. When it reaches 

 the first surface of the film, a known fraction 

 of the light is reflected back. But a large 

 portion of the beam enters the film, reaches 

 its second surface, and is again in part re- 

 flected. The waves from the second surface 

 thus turn back and hotly pursue the waves 

 from the first surface. And, if the thickness 

 of the film be such as to cause the necessary 

 retardation, the two systems of waves inter- 

 fere with each other, producing augmented 

 or diminished light, as the case may be. But, 

 inasmuch as the waves of light are of different 

 lengths, it is plain that, to produce self-ex- 

 tinction in the case of the longer waves, a 

 greater thickness of film is necessary than in 

 the case of the shorter ones. Different colors, 

 therefore, appear at different thicknesses of 

 the film. 



Take with you a little bottle of spirit of 

 turpentine, and pour it into one of the ponds 

 in the Central Park. You will then see the 



flashing of those colors over the surface of 

 the wa^er. On a small scale we produce them 

 thus : A common tea-tray is filled with water, 

 beneath the surface of which dips the end of 

 a pipette. A beam of light falls upon the 

 water, and is reflected by it to the screen. 

 Spirit of turpentine is poured into the pipette; 

 it descends, issues from the end in minute 

 drops, which lise in - uccession to the surface. 

 i On reaching it, each drop spreads suddenly 

 j out as a film, and glowing colors immediately 

 j flash forth upon the screen. The colors 

 change as the thickness of the film changes 

 by evaporation. They are also arranged in 

 zones in consequence of the gradual diminu- 

 tion of thickness from the centre outwards. 



Any film whatever will produce these colors. 

 The film of air between two plates of window- 

 glass, squeezed together, exhibits rich fringes 

 of color. Nor is even air necessary ; the 

 mere rupture of optical continuity suffices. 

 Smite with an axe the black, transparent ice 

 black, because it is transparent and of great 

 depth under the moraine of a glacier ; you 

 readily produce in the interior flaws which no 

 air can reach, and from these flaws the colors 

 of thin plates sometimes break like fire. The 

 colors are commonly seen in flawed crystals ; 

 they are also formed by the film of oxide 

 which collects upon molten lead. It is the 

 colors of thin plates that guide the tempering 

 of steel. But the origin of most historic in- 

 terest is, ns already stated, the soap-bubble. 

 With one of those mixtures employed by the 

 eminent blind philosopher Plateau in his re- 

 searches on the cohesion figures of thin films, 

 \\e obtain in still airabv.bblc twelve or fifteen 

 inches in diameter. You -may look at the 

 bubble itself, or you may look at its projec- 

 tion upon the screen, rich colors arranged in 

 zones are, in both cases, exhibited. Render- 

 ing the beam parallel, and permitting it to 

 impinge upon the sides, bottom, and top of 

 the bubble, gorgeous fans of color overspread 

 the screen, which rotate as the beam is carried 

 round the circumference of the bubble. By 

 this experiment the internal motions of the 

 film are also strikingly displayed. 



Newton sought to measure the thickness of 

 the bubble corresponding to each of these 

 colors ; in fact, he sought to determine gen- 

 erally the relation of color to thickness. His 

 first care was to obtain a film of variable and 

 calculable depth. On a plano-convex glass 

 lens of very feeble curvature he laid a plate of 

 glass with a plane surface, thus obtaining a 

 him of air of gradually increasing depth from 

 the point of contact outwards. On looking at 

 the film in monochromatic light he saw su:- 

 roundingthe place of contact a series of bright 

 rings separated from each other by dark ones, 

 and becoming more closely packed together as 

 the distance from the point of conta t aug- 

 mented. When he employed red light, his 

 rings had certain diameters ; when he em- 

 ployed blue light, the diameters were less. 

 Causing his glasses to pass through the spev/ 



