192 E. S. Ferry — Persistence of Vision. 



respectively, antecedent and consequent — the first including 

 those grand initial movements of debatable cause by which 

 continents are lifted and sometimes deformed or drowned, and 

 the second including the more restricted movements due to 

 loading and unloading. 



So the modern province measures the competence of isos- 

 tasy, the ancient province its incompetence ; the modern Gulf 

 illustrates the magnitude, the ancient Gulf the minitucle, of 

 isostatic deformation as a means in continent making. 



IV. 



Recurring now to the direct and indirect measures of isostasy 

 afforded by the Gulf of Mexico in its present condition and 

 past history, it may confidently be concluded : (1) that the direct 

 data of modern times indicate that deposition and isostatic sub- 

 sidence are not only related sequentially but that under favor- 

 able conditions they are quantitatively equal or sub-equal ; (2) 

 that this measure of isostasy is consistent with the direct data, 

 both quantitative and qualitative, yielded by other noteworthy 

 deposition tracts of the globe; and (3) that the indirect data 

 afforded by the Gulf indicate that isostatic (or consequent) 

 movement alone is incompetent to explain the general conti- 

 nental oscillations recorded in the Neozoic deposits. Thus the 

 Gulf of Mexico yields both maximum and minimum measures 

 of isostasy. 



Art. XXYI. — Persistence of Vision ; by Ervijst S. Ferry. 



[Contributions from the Physical Laboratory of Cornell University, No. 10.] 



Ever since the time of Aristotle, it has been known that 

 when the eye is impressed by light, the sensation persists even 

 after the exciting cause has ceased to act. In his work on 

 dreams, Aristotle describes duration of impressions in the 

 retina and then deduces as the cause of dreams a similar per- 

 sistence of impression on the sensorium of things experienced 

 when awake. The ancients noticed and correctly explained 

 many optical illusions by persistence of vision ; but it was not 

 till twenty centuries after Aristotle that anyone attempted to 

 measure the duration of visual impression. Segner"* measured 

 the duration of the light impression from a spark of a rotating 

 stick and adopted - l second as the probable value. D'Arcy,f 



* De raritate luminis (Gottingen, 1740, pp. 5-8). 



f Memoire sur la dur'ee de la sensation de la vue (1768). 



