UNSEEN LIGHT. 



25 



ILLUMINATED VAPOE8. 



sphere would be visible to the eye, appearing like a vast 

 ball of light a mile in diameter, bright at the centre, and 

 gradually diminishing in brightness from the centre to the 

 outer surface, where the light, by insensible gradations, 

 without any definite boundary, would melt into the dark- 

 ness and disappear. 



It must be understood, however, that this sphere would 

 be thus visible to us, not by means of any of the rays of 

 light which were passing out from the centre to the cir- 

 cumference of the sphere on their regular course, but only 

 by means of that portion of them which was intercepted 

 on the way and reflected to the eye by the solid particles. 



And this brings to our minds a principle of fundamental 

 importance, namely, that no light produces any effect upon 

 our vision except such as passes into the eye. It may pass 

 before us or across our field of view in any quantity and 

 of any intensity without being perceptible to us at all. 

 It is only when it enters the eye, and falls upon the screen 

 called the retina, in the back part of it, that we can have 

 any consciousness of its presence. 



Thus, if such a candle as we have supposed were sur- 

 B 



