884 



ILLUMINATION 



Tho subjoined Table shows the economical ratios of the candles, where the second 

 column gives the quantity of material in grammes which is requisite to produce as 

 'much light as the Carcel lamp : 



These results may be compared with mine given above. A kilogramme, or 1,000 

 grammes =15,440 grains = 2^- Ibs. avoirdupois.' Ure. 



The rule observed in the determination of these questions of illuminating power, 

 is, according to the laws of optics, that the sum of the impinging rays from any 

 source, is inversely as the square of the distance from their source. 



' The numerical estimation of the degrees of intensity of light constitutes that 

 branch of Optics which is termed PHOTOMETBT. 



' If light be a material emanation, a somewhat scattered in minute particles in all 

 directions, it is obvious that the same quantity which is diffused over the surface of a 

 sphere concentric with the luminous points, if it continue its course, will successively 

 be diffused over larger and larger concentric spherical surfaces ; and then its intensity, 

 or the number of rays which fall on a given space, in each will be inversely as the 

 whole surfaces over which it is diffused ; that is, inversely as the square of their radii, 



or of their distances from the source of light Let a candle be placed behind an 



opaque screen, full of small equal and similar holes ; the light will shine through 

 these, and be intercepted in all other parts, forming a pyramidical bundle of rays, 

 having the candle in the common vertex. If a sheet of white paper be placed behind 

 this, it will be seen dotted over with small luminous specks, disposed exactly as the 

 holes in the screen. Suppose the holes so small, their number so great, and the eye 

 so distant from the paper that it cannot distinguish the individual specks, it will still 

 receive a general impression of brightness ; the paper will appear illuminated, and 

 present a mottled appearance, which, however, will grow more uniform as the 

 holes are smaller and closer, and the eye more distant, and if extremely so, the paper 

 will appear uniformly bright. Now if every alternate hole be stopped, the paper 

 will manifestly receive only half the light, and will therefore be only half as much 

 illuminated ; and cateris paribus the degree of illumination is proportional to the 

 number of holes in the screen, or to the number of equally-illuminated specks on the 

 surface ; i.e. if the speck be infinitely diminished in size, and infinitely increased in 

 number to the number of rays which fall on it from the original source of light.' 

 (Herschd.) Seasoning thus, Sir John Herschel proceeds and establishes the following 

 definitions : 



The real intrinsic brightness of a luminous object is the intensity of the light of 

 each physical point in its surface. 



The apparent intrinsic brightness of any object or luminary is the degree of illumi- 

 nation of its image or picture at the bottom of the eye. 



The absolute light of a luminary is the sum of the areas of its elementary portions, 

 each multiplied by its own intrinsic brightness. 



The apparent light of an object is the total quantity of light which enters our eyes 

 from it, however distributed on the retina. 



Various instruments, called photometers, have been devised to measure the illumi- 

 nating power of any bodj' ; these are, all of them, more or less defective, and the 

 results which we obtain with the best of them are merely comparative with each other. 



Bonguer's Photometer consists of two surfaces of white paper, of exactly equal sizo 

 and reflective power, cut from the same piece in contact; these are illuminated, tho 

 one by the light whose illuminating power is to-be measured; and the other by a 

 light whose intensity can bo varied at pleasure by an increase of distance, and can 

 therefore be exactly estimated. The variable light is to bo removed or approached, till 

 the two surfaces are judged to be equally bright, when fche distances of the luminaries 

 being measured, or otherwise allowed for, the measure required is ascertained. 



Rumford's Photometer. Before a screen of white paper, in a darkened room, is 

 placed a blackened cylindrical stick, and the two lights to be compared are so plm-od 

 that two shadows are thrown upon the screen side by side, with an interval between 



