370 Mr. S. Bidweli. On Subjective Colour Phenomena 



the fact that if the thickness of the lines on the disk is much greater 

 than 1 mm., or, more accurately, if it subtends at the eye a greater 

 angle than about one-fifth of a degree, the red and some of the other 

 colours appear only upon the borders of the lines, their inner portions 

 remaining black or grey. 



The true solution, at least as regards the red and the blue, is, I 

 think, to be looked for in certain phenomena attending sudden 

 changes of illumination, which, so far as I have been able to ascertain, 

 have not hitherto been observed. 



The following are a few out of a large number of experiments that 

 have been made during the last four months. They are described, 

 as far as possible, in logical and not in chronological order. Persons 

 unaccustomed to visual observations will not easily perceive some of 

 the effects mentioned. 



Experiment I. 



A circular aperture ^ in. (1'3 cm.) in diameter was made in a sheet 

 of blackened zinc and was covered with thin white writing paper. 

 Diametrically across the aperture a strip of tinfoil -^ in. (1 mm.) 

 wide was attached to the paper. The aperture was closed by a 

 shutter, which could be very rapidly opened by means of a strong 

 spring. The sheet of metal was placed over a window in one side of 

 alight-tight box, inside which at a distance of 1 ft. (30 cm.) from the 

 aperture was an incandescent lamp of 8-candle power with a ground 

 glass bulb. The observations were made at a distance of about 1 ft. 

 from the box, the room being in darkness. 



When the shutter was suddenly opened, several curious phe- 

 nomena appeared simultaneously. The period of their duration was 

 difficult to estimate ; it was probably more than one-twentieth of a 

 second and less than one-tenth. 



(1) Immediately after it was revealed, the small luminous disk 

 first increased in size with extreme rapidity, and afterwards became 

 somewhat smaller, being in its final condition still larger than at the 

 moment of exposure. This effect was more easily seen when the tin- 

 foil strip was looked at : it seemed to become at first much thinner, 

 then thicker again. 



(2) At the moment when the disk was uncovered, a luminous 

 halo, like a broad ring, appeared to start from its margin and spread 

 outwards through a distance of more than an inch (2'5 cm.) in every 

 direction ; then it rapidly contracted and disappeared. The halo was 

 blue or blue-violet in colour, and seemed more sharply defined upon 

 its inner than upon its outer border. 



(3) Contemporaneously with the existence of the halo, the disk 

 was surrounded by a bright red corona, which, like the halo, 

 expanded outwards, and then contracted. There was not, however, 





