264 Dr. S. Bidwell. On Negative After-images, and 



primary colours under similar conditions of illumination ; in particular, 

 the greenish-blue into which bright red appears to be transformed is 

 singularly strong and luminous. This is a matter for some surprise, 

 since it might naturally be expected that the intermittent impressions 

 of the exciting colour, even though not consciously perceived, would 

 be compounded with and tend to enfeeble the complementary hue of 

 the after-image. On the other hand, when the exciting colour is blue 

 or yellow, it is found difficult to obtain a satisfactory pulsative after- 

 image. The complement of blue is an orange-yellow, which is also the 

 hue of the ordinary after-image. But the pulsative image excited by 

 blue, especially if the colour is at all bright, is in most cases an impure 

 pink or salmon of feeble intensity. By using dull greyish-blue pig- 

 ments I have succeeded in obtaining a very fair yellow, which is 

 further improved if a little lamp-black is added to the paint. But in 

 such cases the formation of yellow is no doubt chiefly attributable to 

 the inferior luminosity of the pigment, for a perfectly neutral-grey 

 wash of lamp-black will itself give a yellow image, an effect which is 

 probably due merely to intermittent illumination of feeble intensity. 

 When a yellow pigment is the exciting colour, the hue of the 

 pulsative image is not the complementary blue-violet but a pale 

 purple, only just perceptibly bluer than the subjective purple excited 

 by green. A pulsative image which is really blue has never been 

 obtained from any pigment whatever, the nearest approach being the 

 greenish-blue excited by orange, or the bluish-purple which follows 

 yellow. It has been found equally impossible to obtain either a true 

 red or a true green in the pulsative image. All greens, ranging from 

 yellow-green to green-blue, are transformed into some form of purple, 

 including rose and pink. Purple produces in the pulsative image 

 almost the same kind of blue-green as red, quite different from the 

 pale grass-green colour characterising the ordinary after-image of a 

 purple object. 



The effects observed with the apparatus described above may be 

 shortly summarised in the statement that the pulsative image of a 

 colour in which red predominates is blue-green, that of dull blue is 

 yellow, and that of any other colour (including bright blue) is purple 

 or purplish-grey. In the experiments to be described in the present 

 paper, spectrum colours were used instead of pigments, being blended 

 into uniform mixtures by means of a simple form of Sir W. Abney's 

 well known " colour-patch" apparatus.* 



II. Methods of Experiment. 



Method I. — The arrangement for generating pulsative after-images 

 when the blended spectrum colours are projected upon a screen is shown 



* 'Phil. Trans.,' 1886, Part II, p. 423. 



