THEORY OF ETHER 151 



what is the condition of space through which effects of light 

 and temperature are being propagated. 



The interception of light by an opaque body from the 

 portion of space shadowed by it is readily explained by the 

 propagation of light being rectilinear. A corresponding space, 

 shadowed from thermal radiations, by a body which is ther- 

 mally opaque, may be readily shown to exist by means of a 

 sensitive thermometer. Within this space no change of tem- 

 perature is produced by the radiating body. 



Occasionally it may appear that thermal radiation is 

 greater in an upward direction than others ; but this may be 

 shown to be clearly due to the ascent of air which has been 

 heated by contact with the hot body, and so made less dense 

 than the surrounding air. Radiation may be shown to proceed 

 in a vacuum just as in the air ; while it comes from the sun 

 and stars through vast spaces free of air. 



That effect of radiation which we call light is much more 

 readily and discriminatingly observed than the thermal effect ; 

 and in the following pages greater attention will be paid to 

 the sensations of light in consequence. 



97. Reflexion. When radiation falls upon the surface of 

 any kind of matter, it is in some degree turned back or re- 

 flected. If the surface be irregular, there will be irregular 

 reflexion ; but the smoother the surface, the greater the frac- 

 tion undergoing regular reflexion. It is by the light reflected 

 from bodies that we perceive them, and it is by the manner in 

 which light is reflected from various parts of the same body 

 that we judge of its shape. Without paying attention to the 

 extent of reflexion, we may indirectly verify the following 

 laws, viz. that the angles, which the incident and reflected 

 light make with the normal at the reflecting surface, i.e.. the 

 angles of incidence and reflexion, are equal, and in the same 

 plane. 



A plane mirror, which may be either silvered glass or plate 

 glass, is used, and two cross-wires mounted on a ring serve as an 

 object affording light suitable for the observation of reflexion. 

 If silvered glass be used, a strip in the centre is scraped off 

 so that the cross- wires of a similar ring may be visible at the 



