at the Royal Institution, 1908-1916 747 



the subject of electro-photometrj on the chemical side, developing 

 Edmond Becquerel's discovery (1830) that electric currents attend 

 chemical interactions which are excited by light. The subject is in 

 evident connexion with that of his two previous lectures. Experi- 

 ments were described with cells having poles that were not acted 

 upon chemically and containing a sensitive substance yielding only 

 soluble products. Platinum poles and chlorous acid"^ or chlorine 

 peroxide were selected. 



The amount and the direction of the current, in the case of 

 chlorous acid, were found to be modified by the addition of certain 

 salts and acids ; electrical variations were produced resembling the 

 effects observed when the eye is exposed to light. Salts such as 

 potassium ferro- and f erri-cyanide and especially sodium nitroprusside 

 are strongly affected. Uranium tartrate is one of the most active of 

 organic compounds : a mixture of selenious and suljDhurous acids in 

 admixture with muriatic acid also yields strong currents when exposed 

 to light. It is unfortunate that no further account of this work was 

 pubHshed, the subject being one of great interest and importance. 



It were time that the method developed by McKendrick and 

 Dewar were again made use of and the phenomena of colour vision 

 more fully investigated. The extension of the inquiry to colour- 

 bhnd people is particularly desirable, in view of the work of Abney 

 in recent years ; also in relation to the question of the relation of 

 colour to constitution, to Avhich reference is made in my previous 

 essay. It appears more than probable that the recognition and 

 differentiation of colour is to be correlated with chemical changes in 

 the eyes affecting systems which are in harmony with those of the 

 colour-producing systems ; the study of the electrical response of the 

 eye to the light reflected from coloured substances from this point of 

 view would be of peculiar interest. 



The study of luminous effects has been continued, at intervals, in 

 the Royal Institution laboratory with striking results. In the Friday 

 evening lecture on June 8, 1888, Prof. Dewar drew attention to the 

 phosphorescent phenomena attending the production of ozone. The 

 appearances observed in high vacua have often puzzled spectroscopists, 

 owing to the fact that the chemical changes which take place under 

 such conditions have been unknown. Geissler was the first to dis- 

 cover that phosphorescence is sometimes set up in residual gases in 

 vacuitm tubes. Heat was found to negative such manifestations 

 and several observers came to the conclusion that they were only 

 produced by oxygen compounds. In the lecture, a most striking 

 demonstration of the phenomenon was given by passing a discharge 

 through a current of highly attenuated air streaming into a tall 

 cylinder ; the method is fully illustrated in the abstract of the lecture. 



