n :  a statement in physiology: when a light source of a given size and intensity is just capable of producing visual sensation, reduction of either size or intensity will make it invisible
 
Ricc, Annibale (1844-1919), Italian astrophysicist. A professor of astrophysics, Ricc founded an astronomical and meteorological station on Mt. Etna. For over forty years, he conducted a regular series of direct and spectroscopic observations of the sun, accumulating data on the frequency, position, and development of sunspots and clouds of solar gases and on their influence on terrestrial phenomena. Also interested in geodesy and geophysics, he made measurements in Sicily of the local values of the earth's gravitational and magnetic fields.
 
 
 
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