270 



Mr. Shelford Bid well. On the Negative 



intense or saturated than that acquired by the black ones. Bright 

 diffused daylight produced nearly the same effect as the electric lamp 

 at 4 inches. 



These two experiments together tend to show that, whatever may 

 be the cause of the subjective red border, the nerves of the retina 

 when under its influence behave very much in the same way as if 

 excited by an external red object. 



'Experiment III. 



It was remarked in the former paper (p. 376) that the greenish- 

 blue tint appeared to be of the hue that is complementary to red. 

 Consideration of this fact, in conjunction with the others that have 

 been mentioned, suggested the question whether this tint might not 

 be referable to a negative after-image of the red border. At first 

 sight such a supposition appeared to be an exceedingly improbable 

 one. It is known that under ordinary circumstances a conspicuous 

 after-image can be seen only after prolonged staring at the coloured 

 object, and it seemed incredible that after an excitation lasting for 

 not more than about one-fiftieth of a second there should be deve- 

 loped an after-image of such intensity as to completely overwhelm 

 the primary impression, so that the latter was altogether unnoticed. 

 When, however, the question was tested by experiment, the results 

 were found to be consistent with the conjecture. 



Several cards, measuring about 5 inches by 3, were covered with 

 paper and silk of different colours, and observed through the opening 

 in the rotating disk while the 25-candle power lamp was at a distance 

 of about 6 inches. In every case the original hue entirely vanished, 

 and was replaced by a pale tint of the complementary colour. In 

 order to assist in the recognition of these tints, it was found useful 

 to cover the cards with two different colours, which were approxi- 

 mately complementary to each other. Thus, for example, one-half of 

 a card was covered with red silk, and the other half with green. 

 Under the rotating disk the red portion becomes pale greenish-blue, 

 and the green portion rose-pink, these two hues being recognized by 

 an inexperienced person without the slightest hesitation. The 

 immediate reversal of the colours which takes place when the disk is 

 suddenly stopped is very striking. 



It should be remarked, that there is no blurring of the image upon 

 the retina when objects are viewed through the rotating disk ; even 

 the texture of the coloured silk fabrics is seen quite distinctly. The 

 most suitable speed of rotation is not exactly the same for all 

 persons ; the disk should generally be made to turn some six or eight 

 times per second. 



