128 Mr. G-. J. Burch. 



12'5 cm. focal length. G is a Thorp's replica of Eowland's gratings, 

 15,000 lines to the inch, giving a spectrum of the slit H apparently 

 in the centre of the field of view of the optical system formed by 

 the lenses D, E, F, which constitutes a kind of microscope, with the 

 lens K serving as condenser. Behind K is placed a screen L of suit- 

 able coloured glass, or a train of prisms for illuminating the apparatus 

 with spectral light. 



On rotating the shaft the disc A allows a narrow spectrum of about 

 10 dispersion and 1 wide to appear momentarily, and immediately 

 afterwards a bright flash of monochromatic light fills the field. 



As in my paper on Simultaneous Contrast,* a strip of black 

 material is pasted on the plane side of the lens E to serve as a back- 

 ground for the spectrum, which should exactly cover it. The part, 

 therefore, of the retina on which the image of the spectrum falls is not 

 directly stimulated by the second flash, which fills the rest of the field. 

 The results show, however, that it is indirectly affected as in Dr. Shel- 

 ford Bidwell's experiments. 



With a screen of ruby glass the spectrum corresponds with that seen 

 during temporary red blindness, i.e., the red is missing, and the orange 

 and yellow are replaced by green. With a blue screen composed of a 

 film stained with Prussian blue, and a pale cobalt, or, better, a gelatine 

 film stained with aniline blue, the blue and violet disappear, and the 

 green is weakened, but the red is bright. The addition to these of a 

 yellow glass causes the violet of the spectrum to reappear. And the 

 same effect is produced by a green glass. In a word, the results are 

 exactly similar to those described in Section II, paragraph 3, of my 

 paper on Successive Contrast, the difference between the two experi- 

 ments being that in the new one the retinal area on which the contrast 

 effect, or colour-blindness, is produced, not being exposed to the direct 

 action of the second stimulus, it becomes possible to use a series of 

 rapid flashes instead of a single one, and the effect appears practically 

 continuous. 



But the difference between this and Dr. Shelford Bidwell's experi- 

 ment consists in the use by him of a second stimulus compounded of 

 all the colours, and by me of a second stimulus restricted to one colour. 

 The first step in the discussion must be to ascertain the relation 

 between the results obtained when the second stimulus is white, and 

 when the second stimulus is of the same colour as the first. 



In every experiment I have made, whether with one double flash or 

 with a series of such flashes, the result has been the production of a 

 considerable degree of momentary blindness to the colour or colours 

 composing the second flash of the cycle. And this statement is equally 

 true if the two flashes are of different colours. Whatever colour- 

 sensations are common to the two flashes are wholly or partly blotted 

 * ' Roy. Soc. Proc.,' vol. 67, p. 227. 



