50 The Microscope. 



extreme violet. According to Tyndalhl, the length of an ethereal 

 light Avave of the extreme red would require 36,918, placed end to 

 end, to cover an inch. As the sun's light comes to us from a 

 distance of more than ninety millions of miles, we can perceive the 

 amazing number of waves, and their inconceivable velocity, consid- 

 ering that these waves reach us in the short space of time of 8| 

 minutes, making 168,000 miles per second. 



The number of luminous ether impulses or oscillations neces- 

 sary to produce upon the human retina the impression of red light, 

 is therefore 451 billions per second. Five hundred billions of these 

 oscillations will produce the impression of orange ; 550 billions per 

 second are required for yellow ; 600 billions of impulses will make 

 our retina conceive green light, and in order to produce the impres- 

 sion of extreme violet 789 billions of ether oscillations per second 

 are required. 



When the human mind stands aghast in contemplating the 

 immensity of such etherial wave-motions, it will further increase our 

 wonderment to learn that we have not yet reached the limit of these 

 extraordinary oscillatory wave- motions; for far beyond the violet end 

 of the visible spectrum, rays exist which are still more refrangible, 

 indeed, not perceptible to the unaided eye, but manifesting themselves 

 by chemical and electrical activities. Helmholz says that " these 

 invisible rays, may be made visible if we darken or exclude the 

 more dazzling lighter part of the spectrum from the field of 

 vision." They may become more distinctly visible by placing 

 a fluorescing substance, such, for example, as glass of Uran, or 

 acidified sulph. of chinin, in the ultra-violet part of an object- 

 ive spectrum. Such bodies become luminous in the violet and 

 ultra-violet, and shine in lights of lower refrangibility — for 

 example, in green and blue — and thereby make visible the ultra- 

 violet part of the spectmm with its array of lines. Magnesium 

 light, and that of aluminum, is richer in ultra-violet rays than sun- 

 light. The ultra-violet sun-spectrum shows an extension far beyond 

 the visible spectrum. Its lines are very numerous, running down 

 from H to W. 



We may also show an extension in the ultra-red part of the 

 spectrum. These ultra-red rays are best seen by passing them 

 through prisms of salt crystals; we may thereby elongate the 

 normal spectrum to double its length. The caloric activity of that 

 dark portion of the red end of the spectrum, can be very readily 

 demonstrated. 



