246 PROCEEDINGS OF THE SOCIETY. 



University of Bonn ; was Professor at Heidelberg from 1858 till 1871, 

 when he was appointed to the Chair of Physics at Berlin. This, as well 

 as that at Charlottenburg from 1887, he held till his death in 1894. 



His investigations occupied the whole field of science. In 1851, he 

 discovered and invented the " ophthalmoscope " which has been of 

 inestimable service in medicine. It arose through his attempt to 

 demonstrate to his class the nature of the glow of reflected light some- 

 times seen in the eyes of animals such as the cat. When the great 

 ophthalmologist von Graefe first saw the fundus of the living human 

 eye, with its optic disk and blood-vessels, he exclaimed, " Helmholtz has 

 unfolded to us a new world ! " 



Helmholtz' contributions to physiological optics are of great import- 

 ance. He investigated the optical constants of the eye, measured by his 

 invention, " the ophthalmometer" the radii of curvature of the crystalline 

 lens for near and far vision, explained the mechanism of accommodation 

 by which the eye can focus, within certain limits, discussed the pheno- 

 mena of colour vision, and gave a luminous account of the movements 

 of the eyeballs so as to secure single vision with two eyes. 



In particular he revived and gave new force to the theory of colour- 

 vision associated with the name of Thomas Young, showing the three 

 primary colours to be red, green, and violet, and he applied the theory 

 to the explanation of colour-blindness. 



His great work on Physiological Optics (185G-66) is by far the most 

 important book that has appeared on the physiology and physics of vision. 



Mr. J. W. Gordon said that from its length and character it would 

 be impossible for him to read the paper in extenso, but he thought that 

 as proof copies of it were in the hands of many persons present at the 

 Meeting they would be sufficiently acquainted with its contents to make 

 it easy to follow the argument. He therefore proposed merely to give 

 a summary of its contents. In the first place, it gave a very rough 

 sketch of the theory of diffraction, and proceeded to consider this from 

 a somewhat new point of view, expanding the Helmholtz theory from 

 this position. The paper then went on to deal with the Helmholtz 

 theory, starting with the proof of the "sine law" given in Helmholtz' 

 own paper, in pure mathematical form. This, Mr. Gordon had en- 

 deavoured to set forth under the guise of an experiment. Having 

 proved the sine law, Helmholtz next proceeded to make deductions from 

 it, and in particular to draw the inference that the resolving power of 

 even the most perfect optical system must necessarily stop short at an 

 object which was less than half a wave-length of the light by which it 

 was perceived. Mr. Gordon then in the course of a speech of one hour 

 and twenty minutes set out the points of his own paper, illustrating his 

 remarks by diagrams shown upon the screen and by drawings on the 

 board. Several Microscopes upon the table, to which a mechanical 

 arrangement of moving screens had been adapted, were employed in 

 further illustration of a portion of the subject. 



The President said that the way in which the Fellows of the Society 

 had received this communication made it quite unnecessary to ask them 

 for any expression as to their appreciation of it. Those who had seen 



