RECENT PROGRESS m OPTICS. 



By Prof. W. LeConte Stevens. 



The reviewer who aspires to give an account of recent progress in 

 any department of science is met at the outset by two causes for 

 embarrassment. What beginning shall be selected for developments 

 called recent? What developments shall be selected for discussion 

 from the mass of investigations to which his attention has been called? 

 So rapidly is the army of workers increasing, and so numerous are the 

 journals in which their work is recorded, that the effort to keep up 

 with even half of them is hopeless; or, to borrow a simile employed 

 by the late Professor Huxley, "We are in the case of Tarpeia, who 

 opened the gates of the Roman citadel to the Sabines, and was crushed 

 under the weight of the reward bestowed upon her." 



I have selected a single branch of physics, but one which can scarcely 

 be treated rigorously as single. From the physical standpoint optics 

 includes those phenomena which are presented by ether vibrations 

 within such narrow limits of wave length as can affect the sense of 

 sight. But these waves can scarcely be studied except in connection 

 with those of shorter and of longer period. Whatever may be the 

 instruments employed, the last one of the series through which infor- 

 mation is carried to the brain is the eye. The physicist may fall into 

 error by faulty use of his mathematics, but faulty use of the senses is 

 a danger at least equally frequent. Physiological optics has of late 

 become transferred in large measure to the domain of the psychologist, 

 but he in turn has adopted many of the instruments as well as the 

 methods of the physicist. The two can not afford to part company. 

 If I feel particularly friendly to the psychologist, more so than can be 

 accounted for by devotion to pure physics, it may be fair to plead the 

 influence of old association. If I am known at all in the scientific 

 world, the introduction was accomplished through the medium of physi- 

 ological optics. But, with the limitations imposed, it is not possible 

 even to do justice to all who have done good work in optics. If promi- 

 nence is assigned to the work of Americans, it is not necessary to 



1 Address delivered by Prof. W. LeConte Stevens before the Section of Physics of 

 the American Association for the Advancement of Science, at the Springfield meet- 

 ing, August, 1895. Printed in Nature, No. 1367, Vol. LIII, January 9, 1896. 



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