CHAPTER I. 



A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE. 1 



The law expressed by Stokes 2 in his memoir entitled "The Change 

 of Refrangibility of Light," to the effect that in fluorescence the fluorescent 

 light is always of greater wave-length than the exciting light, has been called 

 in question by Lommel, who pointed out that for certain fluorescent bodies 

 there is an unmistakable overlapping of the regions in the spectrum occupied 

 by the exciting light and by the fluorescence which it produces. Lommel 

 made the further very important statement that for this class of substances 

 the character and composition of the fluorescence spectrum are independent 

 of the wave-length of the exciting light. Lommel's results, in so far as 

 they had to do with the non-validity of Stokes's law, were confirmed by 

 Hagenbach. 3 A few years later Lubarsch 4 published measurements in 

 confirmation of Stokes's law. A later paper by Lommel, 5 in which he 

 described the fluorescence of the so-called chameleon colors, led Hagenbach 

 to new experiments, in the course of which he discovered what he believed 

 to be a source of error in his former measurements, and he reaffirmed the 

 law of Stokes for all such substances. In 1877, Brauner 6 obtained results 

 in confirmation of Lommel's view. In 1879, Lubarsch 7 published further 

 experiments on fluorescence, this time in favor of Lommel's results. Laman- 

 sky' s in 1879 described measurements in confirmation of Stokes's law. In 

 a still later paper Hagenbach 9 returned to the defense of Stokes's law as 

 against Lommel 1 " and Lubarsch, 11 who in the meantime had published 

 further articles dealing with his objections and criticizing Lamansky's 

 method. Wesendonck 1 ' 2 in 1885 made observations with the sun's spectrum, 

 using two concave mirrors and a prism, in the course of which he obtained 

 conclusive evidence that the fluorescence of naphthalin-roth extended to 

 wave-lengths shorter than that of the exciting light. In 1886, Stenger 13 

 took the question up at length. He found that whether he used Hagen- 

 bach's method of illuminating the free surface, Lommel's method of grazing 

 incidence through the side of a flask, or Lubarsch's fluorescent eye-piece, 

 his measurements confirmed Lommel as to the invalidity of Stokes's law 

 but not as to the independence of the fluorescent spectrum from the char- 

 acter of the exciting light. He also made experiments in collaboration with 

 Hagenbach, who was finally converted to the same view. 



It is our purpose in this chapter to describe results obtained by the 

 application of the spectrophotometer to the measurement of the fluo- 

 rescence spectrum of those substances concerning the fluorescence of 



'Physical Review, xvm, p. 403, and xix, p. 18. 



: Stokes, Phil. Trans., p. 463, 1852. 



3 Hagenbaeh, Poggendorff's Ann., 146, pp. 65, 232, 373, 508. 



4 Lubarsch, Poggendorff's Ann., 153, p. 428, 1874. 



'Lommel, Poggendorff's Ann., 159, p. 514, 1876. 



6 Brauner, Wiener Anzeiger, 19, p. 178, 1877. 



'Lubarsch, Wied. Ann., 6, p. 248. 



8 Lamansky, Comptes Rendus, 88, p. 1192, 1879. 



'Hagenbach, Wied. Ann., 18, p. 45. 



10 Lommel, Wied. Ann., 8, p. 244, 1879. 



"Lubarsch, Wied. Ann., 9, p. 665, 1980; also Wied. Ann., 11, p. 68, 1880. 



I2 Wesendonck, Wied. Ann., 26, p. 521, 1885. 



I3 Stenger, Wied. Ann., 28, p. 201. 



