282 POPULAR LECTURES AND ADDRESSES. 



have 800 million millions per second. Beyond 

 that we have something that the eye scarcely 

 perceives does not perceive at all perhaps but 

 which I believe it does perceive, though not 

 vividly ; we have the ultra-violet rays, known 

 to us chiefly by their photographic effect, but 

 known also by many other wonderful experiments 

 which within the last thirty years have enlarged 

 our knowledge of light to a most marvellous 

 degree. We have invisible rays of light made 

 visible by letting them fall on a certain kind of 

 glass, glass tinged with uranium that yellowish- 

 green glass, sometimes called canary glass or 

 chameleon glass. Uranium glass has a property of 

 rendering visible to us invisible rays. You may 

 hold a piece of uranium glass in your hand, 

 illuminated by this electric light, or by a candle 

 or by gas light, or hold it in the prismatic 

 spectrum of white light, and you see it glowing 

 according to the colour of the light which falls 

 upon it ; but place it in the spectrum, beyond 

 the visible violet end, where without it you see 

 nothing, where a piece of chalk held up seems 



