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...chemical laboratory in Clyde Street we made the discovery by noticing a movement of the spot of light on the scale of the galvanometer when the light of a taper fell on the isolated eyeball of a frog. This led to a prolonged investigation which attracted the attention of scientific men in Edinburgh and London and elsewhere. Without our knowledge and about the same time a similar discovery had been made by Holmgren, a physiologist in Upsala. As the investigation had to be conducted in the dark and quiet hours, the time we worked was during the night. Dewar usually came to my house in Castle Terrace soon after 10pm and we then went to the laboratory and worked till two or even three in the morning. This went on for many weeks. In these midnight vigils we were frequently accompanied by friends who came to witness the experiments. On one occasion we had the company of Thomas Huxley who was spending the winter in Edinburgh in charge of the Natural History class during the absence of Wyville Thomson as Director of The Challenger Expedition. Probably the night watchman often wondered what was going on in the laboratory in the hours of early morning - the 'wee short 'oor ayont the twal'. On one occasion one clear night we took part of the apparatus into the street, the galvanometer being in the little room above the laboratory. We had a frog's eye staring at the full moon, to the light of which there was an electrical response. Our joint researches were such as could be most efficiently carried out by a combination of a physicist with a physiologist. In a way it marked an epoch in the lives of both of us. Our investigations followed more of a chemical and physiological nature; Dewar made the chemical substance to be examined and I tested it on animal life. In particular, we examined the physiological action of chinaline and pyridene bases and we laid the foundation which led to the invention by the Germans of not a few artificial chemical compounds, now used in medicine, such as antipyrin, etc. Following the researches of Thomas Fraser and Crum Brown we were on the lines of establishing a relation between chemical composition and physiological action, a view now of great importance in medicine." Dewar and McKendrick made the results of these novel and interesting experiments known to a wider public by the papers which they communicated to The Royal Society of Edinburgh (note 10). On 21st April 1873 they submitted the first of four instalments on "The Physiological Action of Light". As these papers provide the earliest example of the pattern which Dewar followed in all his researches an analysis of their content is not without interest and value to the student of his many varied activities. The opening paragraphs reveal a wide knowledge of what had been written by earlier thinkers on the topic under consideration and indicated what he considered to be the flaws in their arguments. "Numerous hypotheses have been made by physicists and physiologists but up to the present date our knowledge of the subject is without any experimental foundation." Then the several hypotheses by Newton, Vielloni, Seebeck, Young, Du Bois-Reynaud, Draper and Mosier are quoted, none of which had been supported by experiment. Having exposed this deficiency he concludes that there is "obvious need for careful and refined experiment" and in this the two young scientists saw an uncultivated field for research. Their enquiry is to be in two parts, "(1) to ascertain the electro motive force of the retina and (optic) nerve and (2) to observe whether this was altered in amount by the action of light". Using the method devised by Du Bois-Reynaud they had no difficulty in obtaining a strong deflection from the eyes of various rabbits, a cat, a dog, a pigeon, a tortoise, numerous frogs and a goldfish. "The deflection was often so much as to drive the spot of light off the galvanometer scale. In regard to (2) they found Du Bois-Reynaud's galvanometer inadequate so they used Sir William Thomson's "exceedingly sensitive reflecting galvanometer kindly lent to us by Professor Tait". Difficulties, such as the dying of the nerve had to be overcome but the experimenters succeeded in this and they record that "up to this date about five hundred observations were made . . . and we took every occasion to obtain accurate results". As always with Dewar nothing was left to chance. No pains were spared in the thoroughness with which research was planned, the carefulness with which preparations were made and the meticulous care with which the observations were made and recorded before conclusions were drawn. In this case, his conclusions with his usual love of clarity are recorded in nine paragraphs.
(1) The action of light on the retina is to alter the amount of the electro-motive force to the extent of from three to seven per cent of the total amount of the natural current.
(2) A flash of light lasting a fraction of a second produces a marked effect.... NEXT PAGE
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