462 BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 



ami the assimilative, process in plants; and, similarly, tbe vast amonnt 

 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 originated from that used by Lud- 

 wig (1847) in his " Researches on the Circulation;" the mercurial pump, 

 invented by Lothar Meyer, was perfected in the physiological labora- 

 tories of Bonn and Leipsic; the rendering the galvanometer needle 

 aperiodic by dami>ing was first realized by du Bois-Reymond — in all of 

 which cases invention was prompted by the requirements of physiologi- 

 cal research. 



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 organized structures — the eye, the ear, or 

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

 the ])hysioh>gist. Johannes Midler, as is well known, explained the 

 compound 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 (oinmatidia) 

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

 an image is formed which is reversed. Consequently the mosaic theory 

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

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

 subject has been re-investigated, with the result that the mosaic theory 

 has regained its authority. Prof. Exiier* 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 mnh'rstandiug how the whole field of vision is mapped 

 out as coivsistently as it is imaged on our own retina, with the differ- 

 ence, of course, that the jjicture is erect. But behind this fact lies a 

 physical fjuestion — that of the relation between the erect picture which 

 is photographed and the optical structure of the crystal cones which 

 produce it — a question which, although we can not now enter upon it, 

 is quite as interesthig as the physiological one. 



With this history of a theory which, after having been for thirty 

 years disbelieved, has been re-instated by the fortunate combination of 

 methods derived from the two sciences, I will conclude. It may serve 

 to show how, though physiology can never become a part of natural 

 philosophy, the questions we have to deal with are cognate. Without 

 forgetting that every phenomenon has to be regarded with reference 

 to its useful imrpose in the organism, the aim of the physiologist is not 

 to inquire into final causes, but to investigate processes. His question 

 is ever How rather than Why. 



*Exner, "Die Physiologiecler facettirten Aiigen von Krelisen u. Insecten," Leipsic, 

 1891. 



