PHYSICAL FORCES iir 



disappear from a room which is only supplied with light 

 of one amount of refrangibility ; but if ordinary light be 

 admitted to play on any part of the room, the part so 

 illuminated will immediately reappear in its natural 

 colours. Objects seen through a coloured medium or 

 which have had coloured light reflected on them, appear 

 as every one knows to be of the colour of the light so 

 reflected on them or of the colour of the medium through 

 which they are viewed. 



Rays which are less refrangible than the red rays 

 cannot be recognised by the eye. If a body be raised to 

 a temperature of 800 its rays of radiant heat will so 

 illuminate it as to cause it to appear red hot, and at a 

 yet higher temperature such an object may appear 

 white. If the temperature be below 800, however, its 

 heat rays will not cause it to be visible. 



But there are rays of light which are more refrangible 

 than those of violet light. They cannot act on the eye so 

 as to produce any sensation of light, though they have 

 potent effects of another kind. 



It is these highly refrangible rays which act upon 

 photographic plates and they can produce other changes, 

 on which account they are termed actinic, and sometimes 

 chemical rays. Thus there are rays of very different 

 degrees of refrangibility, only the middle series of which 

 serve to illuminate objects and so can be recognised by 

 our sight. The extremes of the whole known series can 

 only be recognised in other modes. But there may be 

 rays which, as yet, have not been recognised in any way, 

 and which may produce effects of which we at present 

 either know nothing, or falsely attribute to other causes. 



Nevertheless not all the rays which come between 

 the actinic and the heat rays always make themselves 

 visible to the human eye. This has been ascertained by 



