526 Mr. A. S. Davis on Recurrent Vision. 



an earnest desire that numerical elective coeflScients might one 

 day be obtained. I submit to chemists the method of ratios as 

 one out of several means of accomplishing that end. The work 

 which Bergman commenced, and which has been now so long 

 intermitted^ may honourably occupy and well be concluded in 

 our own time. It will have its results in a registration of actual 

 or dynamic equivalents, and in the reform of a symbolic system 

 which is every day becoming more disparate from experiment. 



My best thanks are due to Sir C. Taylor, Bart., for the use of 

 his laboratory. 



LXTI. On Recurrent Vision, By A. S. Davis, M.A,^ 



THE following curious phenomenon has not, I believe, been 

 noticed before. If the end of a piece of charcoal be made 

 red-hot in a flame and then waved about in the dark so as to 

 describe an ellipse or circle a few inches in diameter, a blue 

 image of the burning end is seen following the charcoal at a 

 short distance behind it. The space between the charcoal and 

 its blue image is as dark as the surrounding space. The phe- 

 nomenon is much better seen if the charcoal be made bright by 

 being blown upon. The interval of time at which the sensation 

 of blue light succeeds the primary sensation at any point of the 

 retina is about a fifth of a second. This may be ascertained by 

 noticing that when the charcoal is moved round in a circle at 

 the rate of about 100 revolutions per minute, the blue image 

 follows the primary image at a distance of about one third of 

 the circumference of the circle. 



In seeking for an explanation of this phenomenon, it appeared 

 probable that it was related to another phenomenon lately ob- 

 served by Professor C. A. Young, and named by him "recurrent 

 vision ^^f. 



The phenomenon observed by Professor Young is as fol- 

 lows : — ^Yhen the objects in a room are lighted up by a spark 

 from a powerful electric machine (care being taken to screen 

 the eye from the direct light of the spark), it is observed that 

 the illumination is not single, but that the objects appear to be 

 lighted up two or three times in rapid succession. Professor 

 Y'oung found that the interval between tvro successive illumi- 

 nations is about the fifth of a second. He also ascertained that 

 the phenomenon is a subjective one. He does not, however, 

 appear to have noticed whether the colour of the recurrent image 

 difi'ers from the actual colour of the object. 



* Communicated by the Author. 



t See American Journal of Science and Art for April 1S72; and 

 Phil. Mag. for May 1872. - 



