22 PHYSICS 



The conditions with which the physicist has to deal in his study 

 of optics are especially favorable to the development of the scientific 

 imagination, and it is in this field that some of the most remarkable 

 instances of successful speculative work are to be found. The emission 

 theory died hard, and the early advocates of the undulatory theory of 

 light were forced to work up, with a completeness probably without 

 parallel in the history of science, the evidence, necessarily indirect, 

 that in optics we have to do with a wave-motion. The standpoint of 

 optical theory may be deemed conclusive, possibly final, so far as the 

 general proposition is concerned that it is the science of a wave- 

 motion. In a few cases, indeed, such as the photography of the actual 

 nodes of a standing wave-system, by Wiener ,we reach the firm ground 

 of direct observation. 



Optics has nevertheless certain distinctly speculative features. 

 Wave-motion demands a medium. The enormous velocity of light 

 excludes known forms of matter; the transmission of radiation in 

 vacuo and through outer space from the most remote regions of the 

 universe, and at the same time through solids such as glass, demands 

 that this medium shall have properties very different from that of 

 any substance with which chemistry has made us acquainted. 



The assumption of a medium is, indeed, an intellectual necessity, 

 and the attempt to specify definitely the properties which it must 

 possess in order to fulfill the extraordinary functions assigned to it 

 has afforded a field for the highest display of scientific acumen. 

 While the problem of the mechanism of the luminiferous ether has not 

 as yet met with a satisfactory solution, the ingenuity and imaginative 

 power developed in the attack upon its difficulties command our 

 admiration. 



Happily the development of what may be termed the older optics 

 did not depend upon any complete formulation of the mechanics 

 of the ether. Just as the whole of the older mechanics was built up 

 from Kepler's laws, Newton's laws of motion, the law of gravitational 

 attraction, the law of inverse squares, etc., without any necessity of 

 describing the mechanics of gravitation or of any force, or of matter 

 itself, so the system of geometrical relations involved in the con- 

 sideration of reflection and refraction, diffraction, interference, and 

 polarization was brought to virtual completion without introducing 

 the troublesome questions of the nature of the ether and the consti- 

 tution of matter. 



Underlying this field of geometrical optics, or what I have just 

 termed the older optics, are, however, a host of fundamental questions 

 of the utmost interest and importance, the treatment of which de- 

 pends upon molecular mechanics and the mechanics of the ether. Our 

 theories as to the nature and causes of radiation, of absorption, and of 

 dispersion, for example, belong to the newer optics, and are based 



