616 Proceedings. 



glacier, filling the valley, was followed for about eighty miles to a 

 height of 9,000 ft., where it joined the inland ice. On the edge of this 

 inland ice lie found fossil leaves in a bed of sandstone of the Tertiary 

 age. These fossils were not yet worked out, but included Dicotyledons, 

 which pointed to a warmer climate. 



A vote of thanks was acoorded the speakers for their interesting 

 contributions. 



Third Meeting : 19th July, 1904. 



Professor Evans delivered an address on " Photography in 

 Colour." 



The process in hand might, he said, be called " the reproduction of 

 certain effects of light by means of certain other effects of light." What, 

 then, was light? He could not tell them. All he could say was that, 

 just as the clearest views of the phenomena of sound were reached when 

 we imagined them as caused by wave disturbances passing out from 

 sonorous bodies, so was the most comprehensive understanding of the 

 phenomena of light reached if we looked upon it as wave disturbances 

 passing out in all directions from luminous bodies. He went on to 

 explain the science of light, and illustrated his remarks by projecting, by 

 means of a powerful limelight, a white light on the wall, showing that 

 white light was made up of many other colours by inserting a prism and 

 throwing the reflection of the visible spectrum on the wall. He then 

 spoke at some length on the wave theory of light, and illustrated the 

 wave-lengths by means of diagrams thrown on a screen by a lantern. 

 The problem of colour-photography was as old as photography itself. 

 All attempts to solve it could be divided into two groups — they prepared 

 light sensitive surfaces, which retained the colour of the light to which 

 they were exposed, or they produced ordinary photographic pictures, 

 which were coloured and then superposed to obtain the desired effect. 

 The first might be called the direct, and the second the indirect method. 

 The earliest partly successful applications were those of Becquerel, St. 

 Victor, Seebeck, and Poiterin. The two first named covered a highly 

 polished silver mirror with a thin layer of silver-chloride, and exposed it 

 to the light until the delicate surface was converted to the brown sub- 

 chloride. By projecting the solar spectrum on the prepared surface good 

 coloured impressions were obtained. Poiterin substituted paper for the 

 silver substratum, but no other substance had been found that could 

 replace the silver-subchloride. The method and the coloured image 

 given by that substance were not permanent; the image was destroyed 

 by further exposure to light; and, despite numerous experiments, no 

 etiemical had been discovered that could fix the subchlonde without 

 destroying the colour. A light sensitive substance could only be altered 

 by those coloured rays which the substance absorbed ; red light would 

 have no effect on a red body, green rays no influence on a green body. 

 He illustrated these facts by experiments with the lights thrown on the 

 wall, and, after exhibiting slides of photographs of various coloured 

 flowers, showing how the different blossoms had been brought out more 

 prominently by the use of different coloured screens in the photographic 

 process, he went on to explain the different wave-lengths of the lights, and 

 the impressions of photographs taken by the human eye, illustrating the 

 latter with an optical delusion. Continuing, he said that, though the ear 

 could distinguish the different notes in a chord, the eye did not dis 

 tinguish between the colours forming one composite colour. Upon 

 that inability most of the theories of colorisation were based. With 

 the normal eye all the possible colour - sensations could be given 



