SECT. ii. FLUORESCENCE. 61 



neously into parts having more rapid vibrations, which 

 give the harmonics. Professor Stokes of Cambridge, 

 who made this beautiful experiment, computed that the 

 vibrations which produced the fluorescent light were a 

 major or minor third below the pitch or vibrations of 

 the blue light. 



One of the first discoveries of fluorescence was made 

 by Sir John Herschel certainly the first who observed 

 the property in a liquid. He found that the blue light 

 which emanates from all parts of a solution of the 

 sulphate of quinine, especially from its surface, is 

 fluorescent, and that the light transmitted through the 

 liquid, though sensibly like the incident white light, -is 

 no longer capable of producing fluorescence ; it has been 

 deprived of its chemical rays by absorption. 



The chemical rays having been rendered visible by 

 an increase in the length of the periods of vibration, 

 unsuccessful attempts have been made to change the 

 periods of the rays of heat beyond the red end of the 

 spectrum so as to bring them within the limits of vision. 

 The idea of effecting such a change by employing a 

 substance opaque to light, but pervious to heat, is due 

 to Dr. Akin ; but it has since been accomplished by 

 Dr. Tyndall, who, in the course of his experiments on 

 radiant heat, found that a solution of iodine in the bi- 

 sulphide of carbon excludes the most dazzling light, 

 but transmits the rays of heat freely. He employed a 

 mirror, lined in front with silver, to concentrate the 

 rays emitted from the charcoal points of the electric 

 lamp, and interposed a vessel containing the solution in 

 question, so that the rays of heat alone were brought 

 to a focus almost undiniinished. When the solar 

 spectrum was examined, the point of maximum heat was 

 found to be as far beyond the extreme red on one side as 

 the green rays on the other. In the spectrum of the 

 electrical light the point of maximum heat was also 



