52 FKAGMENTS OF SCIENCE. 



than 20)OQ 1 0)0oo of its light at the distance of a foot. 

 Hence, to make the candle-light a mile off equal in 

 power to the non-luminous radiation received from the 

 electric light at a foot distance, its intensity would 

 have to be multiplied by 1,500 X 20,000,000, or by 

 thirty thousand millions. Thus the thirty thousand 

 millionth part of the invisible radiation from the elec- 

 tric light, received by the retina at the distance of a 

 foot, would, if slightly changed in character, be amply 

 sufficient to provoke vision. Nothing could more forci- 

 bly illustrate that special relationship supposed by 

 Melloni and others to subsist between the optic nerve 

 and the oscillating periods of luminous bodies. The 

 optic nerve responds, as it were, to the waves with which 

 it is in consonance, while it refuses to be excited by 

 others of almost infinitely greater energy, whose periods 

 of recurrence are not in unison with its own. 



10. Persistence of Rays. 



At an early part of this lecture it was affirmed, that 

 when a platinum wire was gradually raised to a state of 

 high incandescence, new rays were constantly added, 

 while the intensity of the old ones was increased. Thus, 

 in Dr. Draper's experiments, the rise of temperature 

 that generated the orange, yellow, green, and blue aug- 

 mented the intensity of the red. What is true of the 

 red is true of every other ray of the spectrum, visible 

 and invisible. We cannot indeed see the augmentation 

 of intensity in the region beyond the red, but we can 

 measure it and express it numerically. With this view 

 the following experiment was performed: A spiral of 

 platinum wire was surrounded by a small glass globe 

 to protect it from currents of air; through an orifice 

 in the globe the rays could pass from the spiral and 



