THE EVOLUTION OF OPTICS. 



BY L. A. W. ALLEMAN, M. A., M. D. 



THE first great landmark that the student of the history 

 of optics encounters is the name of Sir Isaac Xewton. Up 

 to his time this science had made a slow and halting prog- 

 ress. The meager knowledge of the ancients which con- 

 sisted in the observed facts of the rectilinear propagation of 

 light, reflection, and refraction had been greatly supple- 

 mented by the labors of Alhazan, an Arabian philosopher, who 

 lived about 1200 A. D. He wrote a valuable text-book on 

 optics, and by an anatomical demonstration proved the ab- 

 surdity of the popular theory of vision viz., that a visual 

 ray was shot out from the eye to grasp the picture of exter- 

 nal objects.* 



After Alhazan follows a long period of inactivity. Then 

 came Roger Bacon, Vitellio, and Kepler, each contributing 

 his share to the fund of knowledge, out of which Xewton 

 was to elaborate a science of optics. 



In 1625 Willebrod Snell enunciated the law governing 

 refraction, which Tyndall terms the " corner-stone of optics," 

 and which connected together all the measurements relative 

 to refraction executed up to his time. This law was " that 

 at any angle of incidence the ratio of the sines of the angles 

 of incidence and of refraction is constant for the same two 

 media, but varies with different media." 



A new life was now infused into the work, and a master 

 mind appears capable of using to best advantage the ma- 

 terial collected ready to his hand. The figure of Xewton 

 rises like a massive peak, supported and sustained, as says 

 Tyndall, by the high table-land of knowledge which had 

 already been attained. 



We can not here attempt even a brief review of the 

 labors of Newton, inviting as is the subject. It is a chap- 

 ter filled with interest, alike to the student of optics and of 

 human progress. It teaches the wonderful possibilities of 



* It is an interesting fact that no one before the time of Aristotle seems to 

 have had the practical sagacity to ask why, if this theory be true, we could not 

 see in the dark. 



