Effect of Pressure on Temperature of Crater of Electric Arc. 377 



strong illumination therefore the sympathetic dark reaction would 

 seem to be least in the case of the red fibres. 



Subjective colours of the same class as those shown by Benham's 

 top, but not nearly so conspicuous, have long been known. Helm- 

 hoi tz* mentions that if a rotating disk with black and white sectors 

 is looked at fixedly, each white sector appears to be reddish along its 

 leading border and bluish along its rear border. He also remarks 

 that these colours are more easily seen upon a disk covered with two 

 spiral bands, black and white, of equal breadth. From these and 

 other observations, Helmholtz concludes that when a point of the 

 retina is exposed to rapid alternations of white light and of darkness, 

 causing successive states of increasing and decreasing excitation, the 

 moment of maximum excitation is not the same for all colours. It 

 has, however, been shown above that in analogous cases the red 

 originates in a portion of the retina which has not been exposed to 

 the direct action of light, while the blue originates in a portion 

 where light has not ceased to act. Helmholtz's supposition therefore 

 does not apply at least to the class of colours at present under con- 

 sideration. 



I have not made any attempt to account for the more feeble 

 colours exhibited by the two intermediate groups of lines in 

 Benham's top, nor for the changes which occur when the speed of 

 rotation is increased. These effects no doubt result, at least in part, 

 from modifications of the phenomena already discussed. But for the 

 present I am compelled to discontinue the experiments on account of 

 the disagreeable and probably injurious effects which they produce 

 upon the eyes. 



" On the Effect of Pressure in the Surrounding Gas on the 

 Temperature of the Crater of an Electric Arc. Correction 

 of Results in former Paper." By W. E. WILSON, F.R.S., 

 and G. F. FITZGERALD, F.R.S. Received November 30, 

 Read December 17, 1896. 



In May, 1895, a preliminary paper by one of the authors was read 

 at the Royal Society, in which is described the apparatus used for 

 these experiments, and the results which were then obtained. 



The primary object of this research was to determine, if possible, 

 whether the temperature of the crater in the positive carbon varies 

 when the pressure in the surrounding gas is changed. 



It has been suggested that the temperature of the crater is that of 



* <Phys.Optik,'23. 

 VOL. LX. ^ r ' 



