Effect of Bodies on Light and Heat. 399 



spar, uranite, and thallene. Paper prepared with these as by paint- 

 ing flowers, or other figures, on the paper with the solution sulphate of 

 quinine, &c. , the figures come out in vivid color when exposed to the 

 invisible rays, while the}' can scarcely be seen in the common light. 

 " From the experiments of Dr. Bence Jones it would seem that thei*e is 

 some substance in the human body that resembles the sulphate of qui- 

 nine, which causes all the tissues of the body to be more or less fluor- 

 escent. The cr} 7 stalline lens of the eye exhibits the effect in a very 

 striking manner. When I plunge my eye into this violet beam I am 

 conscious of a whitish blue shimmer, filling the space before me. This 

 is caused by fluorescent light generated in the eye itself ; looked at 

 from without, the crystalline lens at the same time gleams vividly. M1 



The physical cause of fluorescence is the reduction of the pitch of the 

 tones of vibration, that is, the reduction of their time or rate of vibra- 

 tion per second. The reduction of the length of the waves takes place when 

 the light passes from a rare to a dense medium, which has the effect to reduce 

 the velocity of the progress of the light, but does not of itself affect the rate 

 of vibration of the waves, and by consequence the color of the light. But 

 fluorescence is caused by the reduction of the time or rate of vibration, and 

 is precisely the same sort of phenomenon as the absorption of light by 

 black bodies and its reduction to the invisible tones of heat. Almost 

 all bodies have some such effect upon light. Some absorbing, that is, 

 reducing, some or all the rays of light from the tones of the 44th oc- 

 tave to lower octaves. An opaque body that reduces all, appears black, 

 one that reduces all but the yellow, for example, appears yellow, since 

 that color alone is reflected and reaches the eye. If the body be trans- 

 parent all the colors not transmitted are reduced to heat vibrations. 

 Thus, blue glass is blue because all the rest of the vibrations are re- 

 duced to heat, and blue alone is transmitted, or transmitted in part and 

 reflected in part. This leads to the further consideration of these Meat 

 Tones. If they have the same physical basis as the tones of light, they 

 ought to be subject to the same phenomena of reflection, refraction, 

 double refraction, plane and rotary polarization, magnetization, &c. 

 As the effects of heat cannot be seen, they must be observed by means 

 of an instrument that can feel. Such an instrument is the thermo-elec- 

 tric pile. This consists of a galvanometer connected with a thermo- 

 electric battery, such as that shown in fig. 145. Whenever the ends of 

 the metals are heated to the slightest degree, a current is generated and 

 the needle deflected. Experiments with the nicol prisms with non-lu- 

 minous heat rays, result in the same way as with luminous rays. 

 When the prisms are crossed, the heat passing the first one is inter- 

 cepted by the second, and the needle of the pile remains quiet. But if 



1 Tyndall, Six Lectures on Light, S6. 



