18 Prof. S. P. Langley on Energy and Vision. 



and we can obtain an approximate estimate of this amount of 

 energy as follows : — 



Actinometric measurements made during the progress of 

 the photometric observations showed a solar radiation of 

 1"5 calories per square centimetre per minute. Of this amount 

 of heat the slit (sj), being 3*4 cm. high by O'Ol cm. wide, re- 

 ceived the fraction 0'034. The visible spectrum, from A to 

 H included, according to the bolometer, measures about 21 

 per cent, of the total energy, the absorption of the lower infra- 

 red by the great thickness of glass in the prism being large. 

 We estimate that nearly 20 per cent, had been lost by reflexion 

 before the bolometer was reached. The spectrum formed had 

 a length of 8(3 mm, from A to H. The average energy which 

 passed through the millimetre-apeture of slit ^2 ^^^ there- 

 fore (within these limits and expressed as heat) 



1-5 cab X 0-034 x 0-21 x 0-8 x ^, 



ob 



or approximately xooo^ calorie, let us say 4000 ergs per 

 minute. 



At 1 metre from slit Sg this energy is further spread out over 

 an illuminated area of 28 sq. cm., of which the square centi- 

 metre of fine print, being placed at an angle of 45° with the 

 path of the ray, occupies only about ^q. If a length of one 

 millim. of the standard spectrum receives an average energy of 

 To 0^0 calorie per minute, the actual working part of the screen, 

 consisting of the little square of fine print, will receive at a 

 distance of one metre 4-00V00 calorie per minute. But this 

 by no means gives the amount of energy requisite to produce 

 vision, since the eye is able to receive a distinct visual im- 

 pression in less than one half second of time. We may say, 

 therefore, that a luminous energy of g q-q qIq-q g^ calorie is suffi- 

 cient to give a distinct view of the small square of figures in 

 the brightest part of the spectrum, even after the immense 

 loss of light by absorption and difi'usion in the paper, wliich 

 may amount to |J^ of the whole. 



Even less light is needed to give the bare impression of 

 luminosity. The sensitiveness of the human eye is indeed so 

 extraordinary, that the chief difficulty in measuring its power 

 is to find means for sufficiently reducing the intensity of sun- 

 light which are at the same time capable of even approximate 

 numerical estimation. Out of numerous plans tried, the follow- 

 ing has given the most reliable result: — 



In front of the first slit, in the path of the rays from the 

 siderostat, was placed a plate of glass very lightly smoked. 



