758 Prof. R. W. Wood on 



The experiments which I shall describe in the present 

 paper were carried on in my summer laboratory at East 

 Hampton in 1919 and 1920, and, though they were 

 made with somewhat inadequate facilities, they appeared 

 to establish certain points which were at variance with 

 generally accepted views. 



By operating with a very intense beam of light, obtained 

 by concentrating sunlight with a short-focus lens 6 inches 

 in diameter, sufficient quantities of the transformation 

 products produced by the action of light on fluorescent 

 solutions were obtained to make possible the examination 

 of their optical and chemical properties. The time required 

 to effect the change varies with the nature of the substance : 

 eosine is completely changed in two or three minutes, while 

 rhodamine in the same concentration requires several hours. 

 Usually the decomposition of the fluorescent substance by 

 the action of light yields a coloured non-fluorescent sub- 

 stance which is bleached by the further action of the light 

 to a colourless solution. This makes it impossible to obtain 

 the intermediate substance in a pure state ; but, by choosing 

 the proper concentration and stopping the illumination at 

 the right moment, it was possible to obtain fairly satisfactory 

 results. 



The absorption spectra of the various compounds were 

 photographed by mounting a prismatic cell of quartz filled 

 with the soluiion in front of the slit of a quartz spectro- 

 graph, in which the prism was replaced with a plane 

 grating, and illuminating the instrument with parallel rays 

 of light. In this way we obtain the form of the absorption 

 curve, as well as its location in the spectrum. The angle 

 of the hollow prism was about 10°, and the thin edge (in a 

 horizontal position) was brought into coincidence with the 

 bottom of the slit. The refraction of the fluid prism was 

 compensated by an opposed prism of quartz of about the 

 same angle. 



On PI. XVI. will be found reproductions of some of 

 the photographs. I have called the intermediate coloured 

 substances photo-compounds, for want of a better name. 

 Fluorescein (uranine), for example, which is lemon-yellow 

 by transmitted light, is changed into a non-fluorescent 

 compound (photo-fluorescein), which is orange-red, with 

 an absorption band of a totally different form. The 

 fluorescein band is very steep on the red side, sloping- 

 down gradually towards the region of shorter wave-lengths. 

 The band shown by the photo-fluorescein, however, is more 



