64 Scientific Proceedings, Royal Dublin Society. 



formation of the latent image in the photographic plate suggests itself. 

 Dewar, and subsequently Joly (5), showed that the latent image is produced at 

 extremely low temperatures approaching the absolute zero. This fact, as 

 Joly pointed out, shows that the latent image cannot be produced by the 

 formation of new substances by chemical combination and decomposition, 

 since no such chemical reactions can take place at such low temperatures. 

 Since photo-electricity can be observed at the lowest temperatures, Joly 

 suggested that the latent image is formed by the displacement of electrons, 

 and the ionization so produced revealed when the latent image is converted 

 into the photographic image by development. 



On this theory the action of photographic sensitizers is explained some- 

 what as follows. These bodies, when diffused through, or in contact with, 

 the photographic film, absorb certain wave-lengths, and thus suffer some 

 of their electrons to be displaced. Hence they become ionized, or cause 

 ionization in the surrounding fihn. This action may be caused by light of a 

 frequency which does not directly affect the silver bromide in the film, so 

 that the presence of the sensitizer renders the plate sensitive to light of 

 a colour that otherwise would not affect it. 



According to this view, the photographic film may be used to record the 

 photo-electric properties of the substances admixed with it. In 1874 

 Becquerel (3) exposed eollodium films containing iodide or bronaide of silver 

 with chlorophyll (leaf-extract) admixed to the spectrum. With moderate 

 exposures a marked reduction of silver was found in the film after develop- 

 ment, in the region of the spectrum extending from E (A = b'Zlfi/j.) into the 

 ultra-violet. Longer exposures and intensification revealed photographic 

 action in a group of bands in the green and a strong band in the red between 

 C and B (X = QdGfx/x to A = 687/*/^.) This description coincides closely with the 

 absorption spectrum of leaf-extract according to the recent researches of 

 Willstatter and StoU (9), and may be looked on as strongly suggesting the 

 photo-electric character of chlorophyll, and as a record of the wave-lengths 

 which are effective in displacing its electrons. 



Several researches have been made to ascertain the wave-lengths of light 

 which are utilized in photosynthesis. All investigators are agreed that the 

 wave-lengths between B and C are very effective. Engelmann (4) described a 

 second crest on the curve in the neighbourhood of E (A = 486/x^), and much 

 discussion was raised as to the relative effectiveness of wave-lengths of 

 500/i/j, and less. The work of Kuiep and Minder (6), who worked with screens 

 transmitting equal amounts of energy, showed that equal amounts of light 

 energy of wave-lengths of 620fxfi and more (transmitted through a red screen), 

 or of wave-lengths of 524uju and less (transmitted through a blue screen), 



