L'L'ti Mr. (1. .1. r.urc-h. (hi t1i> S <///./> 



We are much indebted to Mr. Floyer for the great pains he has 

 taken to collect, in Xuliia, the necessary matt-rial for this investigation, 

 and also to Sir W. T. Thiselton-Dyer for having grown the plant at 

 Ke\v, from seed obtained from Egypt. 



'On the Spectroscopic Examination of Colour produced by 

 Simultaneous Contrast." By GEORGE J. BURCH, M.A., 

 Reading College, Reading. Communicated by FRANCIS 

 GOTCH, F.H.S., Professor of Physiology, University of Oxford. 

 Received June 12, Read June 21, 1 900. 



In a previous communication I have described some methods of 

 using the spectroscope to analyse sensations of successive contrast. In 

 those experiments the eye, after having been fatigued by monochro- 

 matic preferably spectral iight, is exposed to a second stimulus, 

 consisting also of spectral light, exciting one or more colour-sensations 

 which may or may not include that fatigued by the primary sensation. 

 The question naturally arises, whether the spectroscopic method 

 might not be applied to problems of simultaneous contrast. 



With this view I made a number of experiments with the Marl- 

 borough spectroscope during the summer of 1897, of which the follow- 

 ing may be mentioned. A piece of thin cover-glass was fixed in front 

 of the eye-piece at an angle of 45 with the optic axis, so as to reflect 

 into the field of view a small complete spectrum furnished by a 3|- 

 inch direct-vision spectroscope. In order that this might be visible 

 against the bright field of the larger spectroscope, a glass disc, with an 

 opaque spot of the required size painted on it, was inserted in the 

 eye-piece close to the diaphragm. 



With this arrangement it was easy to see the effect of contrast upon 

 the smaller spectrum, but the lack of a comparison spectrum made the 

 experiment far less striking than I had anticipated. 



Recently a device has occurred to me by which this difficulty may 

 be got over, namely, the production of simultaneous contrast by dif- 

 ferent colours in the two eyes. 



This method is employed in the well-known experiment by Hering, 

 to show that the apparent alteration of colours by contrast is not due 

 to an error of judgment, but to some real effect produced in the eye 

 itself. 



An ordinary stereoscope is very convenient for this purpose, a square 

 of red glass being inserted on one side of the central partition and a 

 square of blue glass on the other. A small black wafer is then fixed 

 at the centre of each glass, with a white wafer close to the left side of 

 the one on the right-hand glass, and another on the right side of that 





