372 Mr. S. Bidwell. On Subjective Colour Phenomena 



without any intervening paper, no coloured border could be seen, 

 owing, as it seemed, to the glare. 



Experiment III. 



The aperture in the metal plate was again covered with white 

 paper, having a strip of tinfoil across it, and the plate was fixed 

 before the window in the box, as in Experiment I ; a 16-candle power 

 lamp was placed immediately behind it. When the lamp was 

 switched on, the red border was distinctly seen to be backed with 

 greenish -blue, the red itself being much less evident than when the 

 lamp was 18 in. (45 cm.) behind the aperture. 



I have hitherto failed to detect any greenish-bine near the border 

 when the disk was suddenly illuminated by the shutter method of 

 Experiment I, instead of by switching on the lamp.* 



Experiment IV. 



~ The object of this experiment was to ascertain whether the red 

 border could be produced by the sudden accession of light which 

 contained no red constituent. Ten different coloured glasses were 

 successively interposed between the lamp and the aperture with the 

 shutter. In every case when the spectroscope showed that the glass 

 transmitted red light, the tinfoil strip became red, but never other- 

 wise. For example, it reddened with a dark blue cobalt glass, but 

 not with a blue glass which transmitted much more light, but inter- 

 cepted the red end of the spectrum. 



Experiment V. 



The momentary, redness around the edge of the suddenly illumi- 

 nated disk and along the tinfoil strip, as described in the account of 

 the previous experiments, can only be seen by a practised observer. 

 By a different method, however, it can be made quite evident to 

 almost any person whose vision is normal. 



The paper-covered aperture in the box was arranged as before, but 

 the shutter was not used. An incandescent lamp was placed inside 

 the box, and a second lamp outside, at a distance of a few inches 

 from the aperture, the observer's eyes being shaded from it by a 

 screen. The tinfoil strip was on the interior side of the paper, and 

 nothing was seen of it from outside, except when the lamp in the box 

 was alight. 



A rotating commutator was constructed, by means of which 



* [Since this was written, I have found that the greenish-blue may be shown by 

 the shutter method without difficulty if the distance of the lamp from the aperture 

 is suitably adjusted. Dec. 19.] 



