)20 ON LIGHT. 



ing at the hole, will depict on a white screen held at a 

 distance of several feet from the hole a circular image or 

 inverted picture of the sun, which may be considered as 

 the circular base of a cone of rays having the pin-hole 

 for its vertex. In this case, the illumination of the 

 screen, if placed at a great distance, is feeble, and, if 

 near, the circular patch of light inconveniently small. 

 But if instead of a pin-hole, be substituted a convex 

 glass lens of short focus, the whole of the sun-light re- 

 ceived on it will be concentrated in the very small 

 image of the sun formed in its focus, and, diverging 

 thence will spread out into a much wider cone of light, 

 and form a much larger circular and brilliantly illumin- 

 ated area on the screen, aftbrding every facility for the 

 examination of the shadows of objects thrown upon it ; 

 with the additional convenience that by a reflector out- 

 side of the window, the illuminating sunbeam may be 

 thrown horizontally, at whatever time of day the experi- 

 ment is made. 



(104,) The condition essential to the distinct exhibition 

 of all phaenomena of this class that of a very brilliant 

 light emanating from a very small point being thus 

 secured, let an opake body of any form be placed 

 between the point and the screen, so as to cast a shadow 

 on it. It would naturally be expected under such cir- 

 cumstances that the termination of the shadow on all 

 sides should be a clear and sharply-marked outline, 

 separating a uniformly bright space on the outside from 

 a uniformly dark one within, and free from that external 

 gradation from light to darkness which constitutes what 



