368 Mr. S. Bidwell. On Subjective Colour Phenomena 



Note added December 18. 



In addition to the arrangements above described for determining' 

 the capacity of the small condenser, we have also employed the well- 

 known method of charging and discharging the small condenser 

 through a galvanometer by means of a contact-maker driven at a 

 speed of sixty contacts per second by an electrically controlled 

 tuning-fork. By this means a steady deflection of the galvanometer 

 is obtained due to the passage of the rapidly recurring discharges 

 through it. Preliminary observations with this apparatus have con- 

 firmed the above-given value for the dielectric constant of liquid 

 oxygen, and by a modification of it we hope shortly to make a very 

 careful re-determination of the constant. 



" On Subjective Colour Phenomena attending sudden Changes 

 of Illumination." By SHELFORD BIDWELL, M.A., LL.B., 

 F.R.S. Received December 10, Read December 17, 1896. 



The investigation which forms the subject of this paper originated 

 in an attempt to account satisfactorily for the colour phenomena 

 exhibited by Mr. C. E. Benham's "Artificial Spectrum Top," which, 

 when it was brought before the public, about two years ago, excited 

 considerable interest. 



The top consists of a disk of cardboard about 4J in. (1O8 cm.) in 

 diameter, mounted upon a spindle. One half of the disk is painted 

 black; upon the white ground of the other half are drawn four suc- 

 cessive groups of three black lines, having the form of concentric 

 arcs of 45, which are at different distances from the centre, as shown 

 in the annexed figure ; the thickness of the lines is about -$ in. 

 (1 mm.). When the disk rotates, each group of black lines generally 

 appears to assume a different colour. 



The nature of the colours thus developed depends upon the speed 

 of the rotation, and upon the quality and in tensity of the illumination. 

 After several trials, I found that no better results, on the whole, 

 could be obtained than when the disk was illuminated by a 16-candle 

 power incandescent lamp, with a ground glass bulb, at a distance of 

 about 6 in. (15 cm.), and was caused to turn about five times in a 

 second. These, therefore, were adopted as the standard conditions 

 for my experiments, the disk being mounted upon a horizontal axis, 

 driven by an electro-motor, and the speed regulated by comparison 

 with the ticks of an ordinary watch. 



When the disk rotates under the specified conditions and in the 

 direction indicated by the arrow in the figure, the inner group of 



