Proceedings of Societies. 359 



This ratio holds good for all planets ; and it is only in the case of the 

 moon that the error becomes sensible, being then almost exactly one second 

 of arc. 



Monday, 5th March. The Right Rev. Bishop Terrot, in the Chair. 



The following Communications were read : — 



1. On Annelid Tracks in the Equivalents of the Millstone Grits in the 

 south-ivest of the County of Clare. By Professor Harkness. 



See page 278 of the present number of this Journal. 



2. On Superposition. By Professor Kelland. 



The object of this paper was to defend the method of demonstration 

 employed by Euclid from some of the charges which have been at various 

 times brought against it. It particular, it was shown, that the method is 

 not deficient in variety of demonstration of the same fact. This posi- 

 tion was illustrated by the exhibition of twelve totally different demonstra- 

 tions of the problem, " To cut three-fourths of a square into four pieces, 

 which shall form a square." 



3. On the Colouring Matter of the Rottlera Tinctoria. By Dr Anderson. 

 See page 296 of the present number of this Journal. 



Monday, Vdth March. Colonel Madden, Councillor, in the Chair. 

 The following Communications were read : — 



1. Experiments on Colour as perceived by the Eye, with Remarks on 

 Colour-Blindness. By James Clerk Maxwell, Esq., B.A., Trinity 

 College, Cambridge. Communicated by Professor Gregory. 



These experiments were made with the view of ascertaining and regis- 

 tering the judgments of the eye, with respect to colours, and then, by a 

 comparison of the results with each other, by means of a graphical con- 

 struction, testing the accuracy of that theory of the vision of colour, which 

 analyses the colour-sensation into three elements, while it recognises no 

 such triple division in the nature of light, before it reaches the eye. 



The method of experimenting consisted in placing before the eye of the 

 observer two tints, produced by the rapid rotation of a system of discs of 

 coloured paper, arranged so that the proportions of each of the component 

 colours could be changed at pleasure. The apparatus used was a simple 

 top, consisting of a circular plate on which the coloured discs were placed, 

 and a vertical axis. The discs consisted of paper painted with the unmixed 

 colours used in the arts. Each disc was slit along a radius from centre to 

 circumference, so that several could be interlaced, so as to leave exposed 

 a sector of each. The larger discs, about 3 inches diameter, were first 

 combined and placed on the disc, and the smaller, about If inches di- 

 ameter above them, so as to leave a broad ring of the larger discs visible. 



When the top was spun the observer could compare the resulting tint 

 of the outer and inner circles, and by repeated adjustment, perfect identity 

 of colour could be obtained. The proportions of each colour were then 

 ascertained, by reading off on the circumference of the top, which was 

 divided into 100 parts. As an example, it was found on one occasion, 

 that, — 



