320 Intelligence and Miscellaneous Articles. 



formed by a glass tube open outside and entering the interior of the 

 apparatus ; it is into this tube that the metallic electrode is intro- 

 duced which serves for the passage of the discharge. 



The phenomenon of stratification can thus be observed very distinctly. 

 When the coil is in operation, the production of a large quantity of 

 ozone around the apparatus is observed ; I intend to take advantage 

 of this peculiarity in order to construct an apparatus suitable for the 

 production of that gas. — Comptes Rendus, 1871, No. 9. 



ON THE AMOUNT OF TIME NECESSARY FOR VISION. BY OGDEN 

 N. ROOD, PROFESSOR OF PHYSICS IN COLUMBIA COLLEGE. 



In the celebrated experiment of Wheatstone on the duration of 

 the discharge of a Leyden jar, the conclusion was drawn that distinct 

 vision is possible in less than one millionth of a second. The in- 

 correctness of the data on which this conclusion rested was after- 

 ward pointed out in an admirable investigation by Feddersen, who 

 remarks on this point : — " One cannot hereafter assume in optical 

 and physiological experiments that the discharge of a Leyden jar is 

 an instantaneous act ; but at the same time, by the determination 

 of the greatest suitable resistance, it will be possible to limit the 

 discharge to its least possible duration " *. The smallest measured 

 duration obtained by Feddersen was one millionth of a second. 



In an article in Silliman's Journal for September 1871 I show how, 

 by the use of a much smaller electrical surface, I obtained and mea- 

 sured sparks the duration of whose main constituent was only forty 

 billionths of a second. With their light distinct vision is possible ; 

 thus, for example, the letters on a printed page are plainly to be 

 seen; also, if a polariscope be used, the cross and rings around the 

 axes of crystals can be observed with all their peculiarities, and 

 errors in the azimuth of the analyzing prism noticed. There seems 

 also to be evidence that this minute interval of time is sufficient for 

 the production of various subjective optical phenomena — for ex- 

 ample, for the recognition of Loewe's rings (using cobalt glass); also 

 the radiating structure of the crystalline lens can be detected when 

 the light is suitably presented to the eye. 



Hence it is plain that forty billionths of a second is quite sufficient 

 for the production on the retina of a strong and distinct impression ; 

 and as the obliteration of the micrometric lines in the experiment 

 referred to could only take place from the circumstance that the 

 retina retains and combines a whole series of impressions whose 

 joint duration is forty billionths of a second, it follows that a much 

 smaller interval of time will suffice for vision. If we limit the 

 number of views of the lines presented to the eye in a single case to 

 ten, it would result that four billionths of a second is sufficient for 

 human vision, though the probability is that a far shorter time would 

 answer as well, or nearly as well. All of which is not so wonderful 

 if we accept the doctrines of the undulatory theory of light ; for ac- 

 cording to it, in four billionths of a second nearly two and a half 

 millions of the mean undulations of light reach and act on the eye. 

 — Silliman's American Journal, September 1871. 

 * Pogg. Ann. vol. cxiii. p. 453. 



