122 ANNUAL OF SCIENTIFIC DISCOVERT. 



this increase in parallax results a diminution in the earth's dis- 

 tance from the sun, and consequently in tlic distance fjone over in 

 8 minutes 18 seconds by the light; the velocity of light will 

 therefore be reduced to a' little less than 180,420 miles in a second. 

 The next tranf*it of Venus, wliich will hapj^en in 1874, cannot fail 

 to set at rest all doubts which may yet remain on this jjoint, — 

 Delaunay, in Scientific American. 



MECHANICAL EQUIVALENT OF LIGHT. 



Professor Thomsen of Copenhagen has ascertained that the 

 mechanical equivalent of light, of the luminous ratliation as 

 distinct from the obscure radiation, from tlie llanic of tlie French 

 standard bougie is as nearly as possible 1.74 kilogrammetres per 

 minute, being about 1-50 of the mechanical equivalent of tlie 

 total radiation from the same Hame. A writer in " Cosmos" has 

 calculated from this the mcciianical equivalent of the total light 

 of the sun. He finds it to amount to something like that of 1,2;J0 

 septillions of bougies, or to 35 billions of tons lifted a billion of 

 kilometres ])er second — the lifting of 35 billions of tons (French) 

 a billion of kilometres being about equal to lifting the weight of 

 the earth 20 feet. 



COLOR OF SUNLIGHT. 



M. Memorski, of Vienna, confirms M. Brucke's observations, 

 that diffused solar light, instead of being perfectly white, is tinged 

 with red, just as the flames of gas or lamjxs are tinged with yel- 

 low. Difl'used light, received at noon through a cloud}^ sky, de- 

 viates by one twentv'-second part of the chromatic circle from the 

 extreme red of the spectrum toward the violet. The light of burn- 

 ing magnesium, which appears to be so like sunlight, has also a 

 tinge of violet. 



COLORS IN THEIR RELATION TO ARTIFICIAL LIGHT. 



Never select colors in the evening, is an old maxim, whose value 

 can be attested by many a disappointed purchaser, who, ignorant 

 or disregarding tliis advice, and deeming himself the favored pos- 

 sessor of some tint of rare excellence, discovers, on the return of 

 daylight, a color far from equalling his anticipations. The artist, 

 overtaken by darkness, hastens to apply the last touches to some 

 masterpiece ; but the morning light reveals how jjoorly his in- 

 tentions have been realized. The cause of this inconstancy is 

 explained, and a remedy suggested, in a late article in the "Pho- 

 tographic News." 



From the spectral analysis, we learn that the flames of our 

 lamps or gas-lights contain sodium, which, in burning, yields a 

 yellow flame, as strontium gives a red, and iridium a Ijlue flame. 

 Now, when the color blue is illuminated by the yellow ligiit, it ap- 

 pears green ; but if the flame strikes a color complementary to 



