112 A CHAPTER ON LIGHT AND COLOUR. 



the eye endures for no more than 1/7 th of a second. If an object 

 be so faint that the hght which it transmits to the eye in this brief 

 period is not capable of exciting the optic nerve, nothing is seen, 

 but the photographic plate may be exposed continuously for many 

 hours, and so, by the continued action of a feeble excitant, effects 

 may be produced which become visible. Besides affecting sensi- 

 tive plates, these ultra-violet rays possess the power of causing 

 certain changes in a large class of bodies known as Fluorescent 

 bodies. These bodies possess the strange characteristic of absorb- 

 ing radiations of short wave-length, and, by so doing, being excited 

 so as to emit radiations of wave-lengths sufficiently long to affect 

 the eye. Such substances are Sulphate of Quinine, Aesculin, 

 Petroleum Oil, Eosin, etc. 



A somewhat similar phenomenon is manifested by the sul- 

 phides of Barium, Calcium, and Strontium, and some other bodies, 

 which, , when exposed to light for some time, are afterwards able 

 to emit light which can be seen in a dark room. The phenom- 

 enon is known as Phosphorescence, and has a practical applica- 

 tion in the manufacture of luminous paints. 



24. — Besides actual changes of colour such as these, there is an 

 apparent change produced by the motion of the source of light in 

 the line of sight. The cause and the extent of the change are 

 determinable by exactly the same principles as in the analogous 

 case of the change of pitch of sound emitted by a moving body. 

 The phenomenon enables us to extend our knowledge of the 

 motions of the stars, and to measure their motion towards or from 

 the solar system with great accuracy. By this means, too, the 

 violent eruptions connected with the formation of solar spots are 

 rendered capable of measurement, and certain double stars which 

 are too distant to be separated by the telescope are known to be 

 double, from the fact that the spectroscope proves their light to 

 emanate from two bodies moving with different velocities. 



25. — Though not directly connected with the subjects of this 

 article, the above phenomenon, and a further application of the 

 spectroscope to the study of solar physics are too important to 

 leave unmentioned. The sun was observed in 1842, when totally 

 eclipsed, to be possessed of appendages which created much 

 surprise, and a good deal of debate as to their nature and origin. 



