266 The Evolution of Optics. 



conceptions of such astounding magnitude and minuteness 

 that the mind is utterly incapable of representing to itself 

 many of the quantities with which this theory has to deal. 

 Yet the difficulties which the theory presents to our under- 

 standing are no valid arguments against its acceptance, and 

 by that crucial test of all physical theories its ability to 

 explain all the known phenomena, and even to predict re- 

 sults which must follow in given cases the wave-theory of 

 light has established its claim to our acceptance With the 

 final triumph of this hypothesis we bring the science of 

 optics to a comparatively recent date, but we have omitted 

 many considerations which belong properly to such a record, 

 There have grown out from the science of optics, by a natu- 

 ral process of differentiation, new sciences, made possible by 

 its achievements, which rival the parent in interest and im- 

 portance. The employment of lenses in the correction of 

 visual defects, and the combination of lenses and mirrors for 

 assisting the eye to pry into the secrets of the sublimely great 

 and the equally marvelous minute, have played a mighty role 

 in the development of man's intellect. Try to conceive, if 

 you can, the history of scientific thought without the reve- 

 lations of the microscope and telescope. 



Again, spectrum analysis furnishes one of the most beau- 

 tiful examples of the interdependence of kindred sciences. 

 Astronomy had elaborated a theory containing many propo- 

 sitions beyond all hope of demonstration, when its sister 

 science steps in and enables us to subject the most distant 

 luminaries to a chemical analysis much as we would a 

 specimen brought to us in the laboratory We can read in 

 the heavens the history of our own system, we can find 

 worlds in various stages of evolution, and the theory of the 

 astronomer receives the confirmation of the physicist. 



It must not be inferred that in the history of optics 

 proper the record is closed, for it is only with the wave- 

 theory that the real history of optics may be said to begin. 



The wave- theory introduces order and harmony into the 

 phenomena of light. It shows that what was once consid- 

 ered light is but that very little part which the eye is 

 capable of appreciating, of a continuous series of waves ex- 

 tending from the invisible so-called chemical rays to those 

 which our senses interpret as heat. Having thus far tri- 

 umphed, it now proposes to occupy new territory, to bring 

 light and electricity under one common governance, and 

 in this land of promise, in the field of electro-magnetic 



