16 
ANNIVERSARY ADDRESS. 
a certain amount of direct light from a candle falls on to a page of 
a book at a distance of one foot from the candle. Increase the 
distance to two feet. Of course you will now get not more but less 
light on the page, the greater the distance the less the light, the 
ratio is not direct but inverse. But it is not simply inverse; at 
double the distance you do not get half the light. If you have a 
given amount of direct light falling on the page at one foot from 
the candle, you will at two feet have not one-half but one-fourth 
the amount; at three feet you get only one-ninth, at four feet one- 
sixteenth. At ten feet from the candle you will only get on to 
your page from the candle itself one one-hundredth of the direct 
rays of light that it yielded at a distance of one foot. In short, 
light from any source whatever radiates in straight lines and with 
a definite velocity, while the extent to which it illuminates any 
object is inversely as the square of the distance of the object 
from the source of the light. So much for the laws of radiation. 
But in reading the hook, not only do direct rays go from the 
candle to the page, but direct rays which rush from the flame to 
the ceiling or walls of a room are, many of them, reflected, and thus 
we get reflected as well as direct rays of light on our page. Now 
equally simple and equally inflexible laws govern reflection as 
govern radiation of light, laws which operate whenever we ‘ look ’ 
at a thing, that is, whenever we so turn our eyes that the light 
which is being reflected from that thing shall shine into our eyes. 
The things which we term reflectors or mirrors are merely the 
things which unusually, perfectly, and evenly reflect light. In¬ 
genious combinations of reflecting surfaces give the familiar 
kaleidoscope, the so-called ghost (Pepper’s ghost) effects, and form 
the basis of many conjuring tricks, etc. The interesting fact, 
however, in connection with the subject now under consideration 
is this, namely, that no matter by what substance or thing the 
light is reflected—by a book, a mirror, a house, a tree, the ground, 
the moon, or by any one of the millions of objects that present 
themselves to the eye—by all objects the light is reflected not by 
as many millions of modes as there are millions of objects, but 
according to two simple laws. These laws can be understood by 
any persons who are able freely to converse about spaces, magnitudes, 
and numbers. The laws may be thus geometrically expressed : 
Law 1,—the ray of light which is reflected is in the same plane as 
that which shines on the object; while, according to law 2, if a 
ray of light be directed on to an object and be reflected by that 
object, and you raise a perpendicular rod or line from that object 
at the place where the ray strikes, then the angle made by the 
