PROFESSOR AT BONN 171 



Bertrand du 19 octobre,' in which he showed that the motion 

 defined by Bertrand can be due to the combination of a rotation 

 and three rectangular dilatations, and that 'he did not by 

 dilatations mean translations'. 



Lord Kelvin has associated his Theory of the Constitution 

 of Matter with Helmholtz's law that a vortex in a frictionless 

 fluid persists as an invariable quantity. Kelvin sees a funda- 

 mental analogy between the indestructibility of the vortex and 

 the indestructibility of matter. He conceives an atom as a 

 whirl or vortex in the ether, and accounts for the chemical 

 disparity of the atoms on the supposition that we have in them 

 different combinations of vortex rings. 



After Helmholtz had finished this paper, which was intelligible 

 in the first instance only to mathematical physicists, and which, 

 with that on Aerial Vibrations in Tubes, published the following 

 year, was always regarded by Kirchhoff as the author's most 

 important contribution to the subject of mathematical physics, 

 he busied himself in the remaining months of his residence 

 at Bonn with optical and acoustic investigations. 



On July 3, 1858, he read a paper to the Nieder.-Rhein. 

 Gesellschaft, ' On Subjective After-images of the Eye/ which 

 was subsequently extended in Physiological Optics. He had 

 already, in his previous work on Colour- Mixture (under- 

 taken in support of Young's theory of the red-, green-, and 

 violet-perceiving elements of the fibres of the optic nerve) 

 come to the conclusion that the spectral colours are not the 

 most saturated that can occur in visual sensation. In order 

 to settle this last fact he first of all examined Fechner's theory 

 of the subjective after-images of the eye. After looking at 

 a bright object, and then exposing the eye to complete 

 darkness, a positive after-image first appears, i.e. the bright 

 parts of the object appear bright, and the dark are dark ; 

 with uniformly illuminated surfaces, on the contrary, the after- 

 image is mostly negative, i. e. the bright spots of the image 

 appear dark, and the dark, bright. Fechner's explanation is 

 that positive after-images result from persistent excitation of 

 the points of the retina that had been excited by light, negative 

 after-images from fatigue of the same points rendering them 

 less sensitive to new impacts of light ; the strength of illumina- 

 tion of any surface required in order to turn the positive 



