THE PRESIDENTIAL ADDRESS 13 



covered to some constituent which absorbs the green of the spectrum 

 more than the blue : for conservation of energy requires that the 

 green should be absorbed if it is to act on the plate. He then tried 

 staining the plate with coralline red, which has an absorption band 

 in the green, with the expected result. With much prescience he 

 says : ' I think I am pretty well justified in inferring that we are in 

 a position to re?tder bromide of silver sensitive for any colour we choose. 

 Perhaps we may even arrive at this, namely photographing the ultra- 

 red as we have already photographed the ultra-violet.' It was, 

 however, half a century before this far-seeing prophecy was fully 

 realised. The development of the aniline colour industry gave full 

 scope for experiment, but it has been found by bitter experience 

 that dyes which can produce the colour sensitiveness are often fatal 

 to the clean working and keeping qualities of the plate. However, 

 success has been attained, largely by the efforts of Dr. W. H. Mills, 

 of the chemical department of this University, and of Dr. Mees, of 

 the Kodak Company ; and we all see the fruits of it in the photo- 

 graphs by lamplight which are often reproduced in the newspapers. 



It is now known in what direction the molecular structure of the 

 sensitising dye must be elaborated in order to push the action further 

 and further into the infra-red, and the point when water becomes 

 opaque has nearly been reached, with great extension of our know- 

 ledge of the solar spectrum. The spectra of the major planets 

 have also been extended into the infra-red, and this has given the 

 clue as to the true origin of the mysterious absorption bands due 

 to their atmospheres, which had baffled spectroscopists for more 

 than a generation. These bands have been shown by Wildt to be 

 due to methane or marsh gas. Neptune, for example, has an atmo- 

 sphere of methane equivalent to 25 miles thickness of the gas under 

 standard conditions. In this Neptunian methane we have a paraffin 

 certainly not of animal or vegetable origin ; and I venture in passing 

 to make the suggestion that geologists might usefully take it into 

 consideration in discussing the origin of terrestrial petroleum. 



The photographic plate is not the only useful substitute for the 

 human retina. We have another in the photoelectric surface. The 

 history of this discovery is of considerable interest. Heinrich Hertz, 

 in his pioneering investigation of electric waves (1887), made use of 

 the tiny spark which he obtained from his receiving circuit as an 

 indicator. The younger part of my audience must remember that 

 this was before the days of valves and loud speakers. His experi- 

 ments were done within the walls of one room. When he boxed 

 in the indicating spark so as to shield it from daylight and make it 

 easier to see, he found that this precaution had exactly the opposite 

 effect — the spark became less instead of more conspicuous. To 

 express it shortly and colloquially, this action was found to depend 



