INTRODUCTION TO OPTICS. 



Ixxv 



of them. Thus, though its course corresponds with the laws of motion, 

 it does not seem to be influenced by those of gravity ; for it has never 

 been discovered to have weight, though a variety of experiments have 

 been made with a view of ascertaining that point. We are, however, so 

 ignorant of the .intimate nature of light, that an attempt to investigate it 

 would lead us into a labyrinth of perplexity, if not of error. We shall 

 therefore confine our attention to such of its properties as are well ascer- 

 tained. 



To return then to the examination of the effects of the radiation of light 

 from a luminous body ; since the rays are projected in straight lines, 

 when they meet with an opaque body through which they are unable to 

 pass, they are stopped short in their course, for they cannot move in a 

 curve line round the body. The interruption of the rays of light by the 

 opaque body produces therefore darkness on the opposite side of it ; and 

 if this darkness fall upon a wall, a sheet of paper, or any object what- 

 ever, it forms a shadow, for shadow is nothing more than darkness 

 produced by the intervention of an opaque body, which prevents the 

 rays of light from reaching an object behind it. You might suppose 

 from this definition of a shadow, that it would be perfectly black ; but it 

 frequently happens that light from another body reaches the space where 

 the shadow is formed, in which case the shadow is proportionally fainter. 

 This happens if the opaque body be lighted by two candles : if you 

 extinguish one of them, the shadow will be both deeper and more distinct. 

 Yet it will not be perfectly dark, because it is still slightly illuminated by 

 light reflected from the walls of the room, and other surrounding objects. 



There are several things to be 



observed in regard to the form and l &' ' 



extent of shadows. If the lumi- 

 nous body A (Jig. 3) be larger than 

 the opaque body, B, the shadow 

 will gradually diminish in size till it 

 terminate in a point j if smaller, 

 the shadow will continually increase 

 in size, as it is more distant from 

 the object which projects it. The shadow of a figure, A, (Jig. 4) varies 

 in size according to the distance of the several surfaces, B, C, D, E, on 



Fig. 4. 



which if is described. Two lights produce two shadows from the same 

 object. The number of lights (in different directions), while it decreases 

 the intensity of the shadows, increases their number which always corre- 



