AN ILLUMINATED SPHERE. 15 



would be occupied by the radiance would be estimated 

 very differently according to the different tests of the pres- 

 ence of light which we might apply. Still the portion of 

 space that the radiation would fill would in all cases be a 

 sphere, with the luminous source itself in the centre of it, 

 since the limits would be at an equal distance from the 

 centre on every side, whatever might be the test by which 

 the limits were determined. 



Thus every luminous point, the radiation from which is 

 not interrupted on any side, is the centre of an illuminated 

 sphere illuminated in a certain sense, as will be presently 

 explained which sphere is larger or smaller according to 

 the intensity of the light and the transparency of the me- 

 dium surrounding it. In the case of a common candle, and 

 in an ordinary condition of the atmosphere, this sphere 

 might perhaps be a mile in diameter, supposing the limits 

 of it to be determined by the powers of human vision. 



The sphere thus surrounding the luminous point is filled 

 with light that is, filled in a certain sense, which will also, 

 like the sense in which it is illuminated, be presently ex- 

 plained. A light bright enough to be seen at a distance 

 oftfive miles would in the same manner, if its radiance were 

 not obstructed in any direction, form the centre of an illu- 

 minated sphere ten miles in diameter. 



The sense in which this sphere is illuminated or filled 

 with light is this, namely, that if an eye is placed any 

 where within it, and is turned toward the centre, it will see 

 the light that is, there is no part of it in which there 

 would be found any space as large as the pupil of the hu- 

 man eye, and probably not any as large as the area in- 

 cluded by the eye of the smallest insect, that would not 

 furnish rays enough to form an image upon the retina so 

 as to produce vision. 



And here I must pause a moment to explain how it is 



