SIX LECTURES ON LIGHT. 



for interference, when the waves are reduced 

 by the analyzer to a common plane. A 

 highly beautiful and important source of 

 chromatic phenomena is thus revealed. 

 Placing the plate of spar between the crossed 

 prisms we have upon the screen a beautiful 

 system of iris rings surrounding the end of 

 the optic axis, the circular bands of color 

 being intersected by a black cross. The 

 arms ^f th : s cross are parallel to the two 

 directions of vibration in the polarizer and 

 analyzer. It is easy to see that those rays 

 whose planes of vibration within the spar 

 coincide with the plane of vibration of either 

 prism, cannot get through both. This com- 

 plete interception produces the arms of the 

 cross. With mono-chromatic light the rings 

 would be simply bright and black the 

 bright rings occurring at those thicknesses of 

 the spar which cause the rays to conspire ; 

 the black rings at those thicknesses which 

 cause them to quench each other. Here, 

 however, as elsewhere, the different lengths 

 of the light-waves give rise to iris-colors 

 when white light is employed. 



Besides the regular crystals which produce 

 double refraction in no direction, and the 

 nniaxal crystals which produce it in all direc- 

 tions but one, Brewster discovered that in a 

 large class of crystals there are two directions 

 in which double refraction does not take 

 place. These are called biaxal crystals. 

 When plates ot these crystals, suitably cut, 

 , are placed between the polarizer and analyzer, 

 Ihe axes are seen su -ounded, not by circles, 

 but by curves of ano her order and of a per- 

 fectly definite mathem* : cal character. Each 

 band, as proved experimentally by Herschel, 

 forms a lemniscata ; but the experimental 

 proof was here, as in numberless other cases, 

 preceded by the deduction which showed that, 

 according to the undulatory theory, the bands 

 must possess this special character. 



I have taken this somewhat wide range 

 over polarization itselt and over the phenom- 

 ena exhibited by crystals in polarized light, 

 in order to give you some notion of the firm- 

 ness a. id completeness of the theory which 

 grasps them all. Starting from the single 

 assumption of transverse undulations, we 

 first of all determine the wave-lengths, and 

 find all the phenomena of color dependent 

 on this element. The wave-lengths may be 

 determined in many independent ways, and, 

 when the lengths so determined are compared 

 together, the strictest agreement is found to 

 exist between them. We follow the ether 

 into the most complicated cases of interac- 

 tion between it and ordinary matter, "the 

 theory is equal to them all. It makes not a 

 single new hypothesis ; but out of its original 

 stock of principles it educes the counterparts 

 of all that observation shows. It accounts 

 for, explains, simplifies the most entangled 

 cases ; corrects known laws and facts ; pre- 

 dicts an 1 discloses unknown ones ; becomes 

 the guide of its former teacher Observation ; 



and, enlightened by mechanical conceptions, 

 acquires an insight which pierces through 

 shape and color to force and cause. "* 



But, while I have thus endeavored to illus- 

 trate before you the power of the undulatory 

 theory as a solver of all the difficulties of 

 optics, do I therefore wish you to close your 

 eyes to any evidence that may arise against 

 it ? By no means. You may urge, and 

 justly urge, that a hundred years ago another 

 theory was held by the most eminent men, 

 and that, as the theory then held had to 

 yield, the undulatory theory may have to 

 yield also. This is perfectly logical ; but let 

 us understand the precise value of the argu- 

 ment. In similar language a person in the 

 time of Newton, or even in our time, might 

 reason thus: " Hipparchus and Ptolemy, 

 and numbers of great men after them, be- 

 lieved that the earth was the centre of the 

 solar system. But this deep-set theoretic 

 notion had to give way, and the theory of 

 gravitation may, in its turn, have to give 

 way also." This is just as logical as the first 

 argument. Wherein consists the strength of 

 the theory of gravitation ? Solely in its com- 

 petence to account for all the phenomena of the 

 solar system. Wherein consists the strength 

 of the theory of undulation? Solely in its 

 competence to disentangle and explain phe- 

 tnomena a hundred-fold more complex than 

 those of the solar system. Be as skeptical, 

 if you like, regarding the undulatory theory ; 

 but if your skepticism be philosophical, it 

 will wrap the theory of gravitation in the 

 same or greater doubt, f 



LECTURE V. 



Range of vision incommensurate with Range of Radi- 

 ation: The Ultra- Violet Rays: Fluorescence: 

 Rendering Invisible Rays visible: Vision not the 

 only Sense appealed to by the Solar and Electric 

 Beam : Heat of Beam : Combustion by Total Beam 

 at the Foci of Mirrors and Lenses: Combustion 

 through Ice-Lens: Ignition of Diamond: Search 

 for the Rays here effective : Sir Villiain Herschel's 

 Discovery of Dark Solar Rays: Invisible Rays the 

 Basis of ttie Visible : Detachment by a Ray-Filter 

 of the Invisible Rays from the Visible; Combustion 

 at Dark Foci: Conversion of Heat- Rays into 

 Light-Rays: Calorescence : Part played in Nature 

 by Dark Rays: Identity of Light and Radiant 

 Heat: Invisible Images: Reflection, Refraction, 

 Plane Polarization, Depolariz.ition. Circular Polar- 

 ization, Double Refraction, and Magnetization of 

 Radiant Heat. 



THE first question that we have to con- 

 sider to-night is this : Is the eye, as an organ 

 of vision, commensurate with the wnole 

 range of solar radiation is it capable of re- 

 ceiving visual impressions from all the rays 

 emitted by the sun? The answer is nega- 

 tive. If we allowed ourselves to accept for a 



* Whewell. 



t The only essay known to me on the Undulatory 

 Theory, from the pen of an American writer, is an 

 excellent one by President Barnard, published in ths 

 Smithsonian Report for 1862. 



