400 History of Luminescence 



in color of tungstic acid from yellow to green and of cobalt chloride 

 from red to blue on heating, effects which are due to loss of water 

 and not related in any way to fluorescence. 



The negative fluorescence of Tyndall and Akin, which led to a 

 priority controversy, was no more than the demonstration that by 

 infrared rays of high intensity, matter could be heated to the tem- 

 perature where visible light is emitted. These men heated, among 

 other things, a spiral of platinum to incandescence by the concen- 

 trated invisible rays of the sun, filtered through iodine dissolved in 

 carbon bisulphide. ^° Akin used the word " calcescence," from the 

 Latin word for lime {calx) , since lime is especially bright on heat- 

 ing, while Tyndall suggested that " calorescence " be used for nega- 

 tive fluorescence. However, none of these words has any real 

 significance, although they started a bitter controversy between 

 Emsmann, Akin and Tyndall. Bohn (1867, 1868), pointed out 

 the error of the conception. 



FLUORESCENCE DISTRIBUTION AND FLUORESCENCE ANALYSIS 



The flood of early papers describing new cases of fluorescence is 

 too vast to be included in any history. Many were merely concerned 

 with the fact that one or another plant or animal extract or organic 

 compound is fluorescent, but some were extensive publications such 

 as that of E. Hagenbach (1869) on " leaf green." There appeared 

 to be a wide distribiuion of fluorescent substances in the living 

 world. C. B. Greiss (1861, 1864) was moved to write: "All parts 

 of all plants contain fluorescent substances." It is not surprising 

 that H. von Helmholtz in 1855 should examine the retina of a 

 human eye from a cadaver, in connection with his study of sensi- 

 tivity of the eye to radiation. He found that the retina was in fact 

 fluorescent, and pointed out that the sensation of violet in this 

 region of the spectrum was modified by the greenish white fluores- 

 cence. Retinal fluorescence was confirmed for the living retina by 

 M. von Bezold and G. Englehardt (1877) . The cornea and lens of 

 the eye are also fluorescent, as all those who have been in a dark 

 room with intense ultraviolet light will realize. The " haze " which 

 can be observed under these conditions is quite striking. E. Briicke 

 (1845, 1846) had noticed that lens and cornea were opaque to 

 ultraviolet light, but he worked before Stokes' paper appeared. 



Although Brewster described the red fluorescence of chlorophyll 

 in 1833, the red fluorescence of a derivative of the related animal 

 pigment, hemoglobin was not observed until 1868, when J. L. W. 



^^ Tyndall discoveied this filter in 1862. 



