Imprisonment of Radiation by Total Reflexion, 449 



Prof. Michelson's gratings, in their 75-foot spectrograph 

 (Tower telescope), which is superior to those which I made 

 at E. Hampton. Prof. Michelson's grating shows even more 

 than full theoretical resolving power, and is superior to the 

 one with which my work was done. 



XLVIII. On the Imprisonment of Radiation by Total Reflexion. 

 By H. W. Wood, Professor of Experimental Physics, 

 Johns Hopkins University, and Adams Research Fellow of 

 Colum bia Un i versity * . 



MY attention was recently drawn to a somewhat surprising 

 circumstance in connexion with fluorescence by 

 Dr. Cooper Hewitt. As is well known, he has now perfected 

 his rhodamine fluorescent reflectors for the mercury arc to 

 such a point that the red fluorescent radiation, plus the direct 

 light of the arc, makes a fairly good imitation of daylight. 

 He found that if the celluloid film stained with rhodamine 

 was backed by white paper the fluorescence was many times 

 brighter than when a backing of silver paper was used. 

 I verified the matter by depositing silver chemically over a 

 small circular area in the centre of a white porcelain plate, 

 and flowing the whole with the rhodamine solution, or with 

 water containing a little fluorescein. Notwithstanding- the 

 very high reflecting power of the polished silver film, the 

 intensity of the fluorescence over the circular area was so feeble 

 that it appeared almost black in contrast with the brilliant 

 fluorescence displayed over the surface of the porcelain. 

 The plate was illuminated with the mercury arc in the case 

 of the rhodamine, or with that of the carbon arc, passed 

 through dense cobalt glass in the case of the fluorescein, the 

 experiment being well suited for lecture purposes. If we 

 strip off the coloured celluloid film from the porcelain plate 

 we find its fluorescence becomes even less brilliant than that 

 of the portion backed by silver. The action of the whit® 

 porcelain surface in raising the intensity of the fluorescence 

 of the film covering it appeared a little puzzling at first 

 It was noticed, however, that the films which had been 

 stripped from the surface glowed brilliantly along the edges y 

 and this circumstance gave me the clue to the action of the 

 matt reflecting surface. The greater luminosity of the edges 

 was evidently due to the circumstance that more radiation 

 was able to escape here than through an equal area of tlie 

 surface of the film. The action of the matt surface will be 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 25. No. 148. April 1913. 2 1 



