30 EEPOET— 1893. 



There is, tberefore, no doubt as to the advantages which physiology 

 derives from the exact sciences. It could scarcely be averred that they 

 would benefit in anything like the same degree from closer association 

 with the science of life. Nevertheless, there are some points in respect ot 

 which that science may have usefully contributed to the advancement of 

 physics or of chemistry. The discovery of Graham as to the characters 

 of colloid substances, and as to the dififusion of bodies in solution through 

 membranes, would never have been made had not Graham ' ploughed,' so 

 to speak, ' with our heifer.' The relations of certain colouring matters to 

 oxygen and carbon dioxide would have been unknown, had no experiments 

 been made on the respii'ation of animals and the assimilative process in 

 plants ; and, similarly, the vast amount of knowledge which relates to the 

 chemical action of ferments must be claimed as of physiological origin. 

 So also there are methods, both physical and chemical, which were 

 originally devised for physiological purposes. Thus the method by which 

 meteorological phenomena are continuously recorded graphically, origi- 

 nated from that used by Ludwig (1847) in his ' Researches on the Circula- 

 tion ' ; the mercurial pump, invented by Lothar Meyer, was perfected in 

 the physiological laboratories of Bonn and Leipzig ; the rendering the 

 galvanometer needle aperiodic by damping was first realised by du Bois- 

 Reymond — in all of which cases invention was prompted by the require- 

 ments of physiological researcb. 



Let me conclude with one more instance of a diiferent kind, which 

 may serve to show how, perhaps, the wonderful ingenuity of contrivance 

 which is displayed in certain organised structures — the eye, the ear, or 

 the organ of voice — may be of no less interest to the physicist than to the 

 physiologist. Johannes Miiller, as is well known, explained the com- 

 pound eye of insects on the theory that an erect picture is formed on the 

 convex retina by the combination of pencils of light, received from 

 different parts of the visual field through the eyelets (ommatidia) 

 directed to them. Years afterwards it was shown that in each eyelet an 

 image is foi-med which is reversed. Consequently, the mosaic theory of 

 Miiller was for a long period discredited on the ground that an erect 

 picture could not be made up of ' upside-down ' images. Lately the 

 subject has been reinvestigated, with the result that the mosaic theory 

 has regained its authority. Professor Exner ^ has proved photographically 

 that behind each part of the insect's eye an erect picture is formed of 

 the objects towards which it is directed. There is, therefore, no longer 

 any difficulty in understanding how the whole field of vision is mapped 

 out as consistently as it is imaged on our own retina, with the difference, 

 of course, that the picture is erect. But behind this fact lies a physical 

 question — that of the relation between the erect picture which is photo- 

 graphed and the optical structure of the crystal cones which produce it — 



' Exner, Die Pliysiologie dcr facettirten Augen von Krehsen u. Insecten, Leipzig, 

 1891. 



