208 



Mr. J. Burcli. On the Relation of 



This gives the complementary colour, i.e., the direct spectrum, as 

 Darwin calls it, of all the colours save that excited by the first 

 stimulus. 



(2.) The second stimulus may excite the same sensation as the first, 

 hut less strongly. 



This gives a black after-image. 



(3.) The second stimulus may excite two or more colour-sensations, 

 including that of the first stimulus. 



The colour of the resulting image is that of the second stimulus minus 

 the first. 



(4.) The second stimulus may excite one or more sensations, none of 

 which were included in the first stimulus. 



The colour of the resulting image is that of the second stimulus plus 

 an admixture, usually small, of the first. 



This scheme is covered by the experiments described in the following 

 pages. They are so arranged as to require but little special apparatus, 

 and to employ spectral colours by direct observation. Some little care 

 must be taken to adjust the relative intensity of the two stimuli 

 <3orrectly, and to effect the change from one to the other as suddenly as 

 possible. 



1. After fatiguing the retina by the spectrum, to observe a uniform white 

 light. 



This is most easily done by means of a low-power spectroscope with 

 ;a reflected-scale tube. Unscrew the cap containing the scale, and place 

 ^ mirror so as to reflect white light from the sky into the tube. Cover 

 the open end of the scale tube by a black card held in the right hand, 

 and have a similar card in the left hand in readiness to cover the slit of 

 the spectroscope. Let the slit be fairly wide so ae to give a rather 

 bright spectrum. Look steadily at^ the spectrum for half a minute, 

 keeping the eye fixed on the intersection of the cross wires, and then 

 suddenly cover the slit and uncover the scale tube. A complementary 

 spectrum will be seen, brilliantly defined, for a fraction of a second. 

 To myself, by daylight the spectral red is replaced not by green, but by 

 blue, and the complementary of green is a pinkish purple, but by lamp- 

 light the complementary of red is green, and that of green is red. 

 The advantage of this mode of experimenting is that it utilises existing 

 apparatus. 



2. After fatiguing the retina by the spectrum to observe a less intense 

 spectrum. 



The phenomenon of successive contrast is shown by the preceding 

 method in its least simple form. To analyse it, the efiect of retinal 

 fatigue by each spectral colour on the perception of that same colour 

 must be determined. This may be done by focussing with a lens A 

 i(fig. 1) a glow lamp B on the slit C of an ordinary spectroscope, and at 

 the same time illuminating it by a second glow lamp D placed between 



