274 IN STARRY REALMS. 



these would be 30 times as long as this tiny wave of light. 

 At the other end of the visual spectrum is the extreme 

 violet, the wave lengths are only about half as long, while 

 in that ultra-violet region occupied by rays incompetent 

 to excite any nerves possessed by our retina, but acutely 

 perceptible by the peculiar sensibility of the photographic 

 plate, the waves are shorter still. 



Light travels across space with a tremendous speed of 

 185,000 miles per second. It gives us a wonderful im- 

 pression of the subtlety of the ether to think that waves, 

 of which tens of thousands are comprised within the 

 length of a single inch, can oscillate with such astonish- 

 ing rapidity that in a single second of time 185,000 miles 

 shall be travelled, each mile having 1,760 yards, each yard 

 having three feet, each foot twelve inches, and each inch 

 some thirty or forty thousand wave lengths. 



A remarkable circumstance must now be mentioned ; it 

 has been discovered that rays of every colour travel with 

 the same speed. If a ray of blue and a ray of red be 

 started at the same moment they will require precisely the 

 same time to travel a million miles or any other distance. 

 If in the depths of space a blue star and a red star simulta- 

 neously sprang into existence, we should become aware of 

 the two creations at the same instant, provided the stars 

 were equally distant from us. Hence we are able to con- 

 clude the time of vibration of a wave of any given colour. 

 The vibrations of the ether as it conveys a red ray of light 

 are not so rapid as those which convey a ray from the 

 violet end of the spectrum ; or, to speak a little more cor- 

 rectly, when certain nerves on the retina are tingling with 

 so many vibrations a second, the brain is conscious of a 



