DIVERGENCE OP KAYS. 23 



must return to the illumined sphere, which, I have said, 

 always surrounds every luminous point, so far as there is 

 no object intervening to intercept the rays. 



This sphere, as has already been said, is filled with light 

 in the sense that in no part of it can an eye be placed 

 where there will not be rays enough, if the eye is turned 

 toward the luminous point, to enter the pupil and form an 

 image of the source of the light on the retina, as John 

 formed an image of the candle on his card by the lens. 

 And it is in this sense only that the space within the 

 sphere is illumined, namely, that it is completely filled 

 with rays of light proceeding in close proximity to each 

 other from the centre to the circumference. 



These rays, it is true, diminish in intensity, in some mys- 

 terious way, as they proceed from the centre to the circum- 

 ference ; but, in whatever way this diminution of intensity 

 is eifected, it is not done by a separation of the rays from 

 each other as they diverge, so as to leave some parts of the 

 space empty. 



Radiation from a luminous point is, indeed, often in books 

 represented by lines diverging from each other as tliey re- 

 cede from the centre, and this, however closely the lines 

 are together in the centre, gives us the idea of a necessary 

 separation between them toward the outer portions. But 

 we must not imagine that the diminution in the intensity 

 of light, as the distance from the source increases, is pro- 

 duced by any separation of the rays. Exactly how we are 

 to picture this diminution of intensity to our minds it is 

 difficult to say, but it is certain that it is not the result of 

 the separation of lines of radiance from each other as they 

 recede, leaving intervals between them dark. 



In describing the phenomena we use the word rays, and 

 we represent the radiation by lines ; but we must conceive 

 of it, so far as we can, as homogeneous throughout, and as 



