TRANSACTIONS OP THE SECTIONS. 5 



the perfection of every part. There was not anything new given in the discoveries 

 or ehcited in the discussion which ensued, but from the crowded state of the Section, 

 it appeared to excite much interest to the end. 



On the 3/ixture of Homogetieous Colours. 

 By Professor Helmholtz, Konigsberg. 



The author pubUshed a year ago experiments on the mixture of homogeneous 

 coloured light, which seemed to prove that there are only two colours in the solar 

 spectrum capableof being combined into white, namely, yellow and indigo. He has 

 repeated these experiments, following another method, similar to that lately described 

 by M. Foucault, for obtaining larger fields equally dyed with the mixture of two 

 homogeneous colours, and has found that there are more pairs of complementary 

 colours in the spectrum. These colours are situated at both ends of the spectrum, 

 ~on one side from red up to a yellow shade, a little greenish, — on the other side 

 from violet up to a blue shade, also a little greenish. The shades, however, in the 

 middle of the spectrum, in which the green preponderates, cannot give white with 

 any other homogeneous colour. Their complement is purple, and must be com- 

 pounded by violet and red. The complementary colour of red is greenish-blue, — of 

 orange, sky-blue, — of yellow, indigo, — of greenish-yellow, violet. 



The author found, moreover, that the complementary colours are arranged in the 

 spectrum in a most irregular manner. As the breadth of the differently-coloured 

 bands in prismatic spectra depends not only on the wave-length, but on the sub- 

 stance of the prism, he refers the following results to interferential spectra, where 

 the distance of two colours is proportional to the difference of their respective 

 wave-lengths. If you pass with an equal velocity through the different colours of 

 such a spectrum, the shade is altered very slowly at both its extremities on the red 

 and violet ; but in those parts where the complements of red and violet are placed 

 in the greenish -yellow and greenish-blue, the shade alters very rapidly, so that the 

 distance of extreme red and golden-yellow is about ten times greater than the 

 distance of their complementary colours, greenish-blue and sky-blue. 



The author observed two circumstances in these experiments which had prevented 

 him in his former experiments from finding other complementary colours than yellow 

 and indigo. At first, according to the pecuhar distribution of complementary shades 

 in the spectrum, the said colours were able to give a larger white spot than the 

 others. Secondly, it appeared to be very diflicult to the human eye, which is not 

 quite achromatical, to find and to keep the right focal length for objects illuminated 

 by two kinds of homogeneous rays of very different refrangibiUty . Indigo and yellow 

 are of less different refrangibility than any other pair of homogeneous complementary 

 colours, and are therefore easily combined. Others, as red and greenish-blue, on 

 the contrary, are united in the same field of the retina with great difficulty. 



Finally, the author gave some remarks on the best method for bringing the whole 

 variety of colours into a system. He stated that Newton's coloured disc appeared 

 to be the most simple and complete manner. Some points, however, are to be 

 changed. First, not only the seven principal colours of Newton must be arranged 

 on the margin of the disc, but the whole infinite number of them existing in the 

 spectrum, so that complementary colours are placed on the opposite ends of the same 

 diameter. Secondly, the two ends of the spectrum cannot meet together, but must 

 be separated by an interval, where the complementary colour of the green shades, 

 namely purple, is to be intercalated. The commonly received theory of three prin- 

 cipal colours includes a restriction of Newton's method, contradictory to the author's 

 former experiments. 



On the Distribution of Electrical Currents in the Rotating Disc ofM. Arago. 

 St/ Professor Matteucci, Pisa. 

 After the discovery of the induction between the electro-magnet and the closed 

 conducting circuit, Faraday conceived the idea of applying the extremities of a 

 galvanometer upon a disc of copper revolving in the neighbourhood of a magnet. la 

 this way he found the electric currents, which were developed by the induction 

 of the magnet, upon the disc, of which the points change successively according to 



