218 



Mr. G. J. Burch. On the Production of 



heliostat. I did not therefore make any observation with blue or 

 violet light. 



As may be imagined, it is necessary to use a larger lens with moon- 

 light than with sunlight. In practice I have found an ordinary reading 

 lens, of 4 inches diameter, sufficient. To obtain the full intensity of illu- 

 mination, the focal length should be such that the moon's image may 

 be not smaller than the pupil of the eye. 



There are two points of interest in connection with this experiment. 

 The first is that the illumination of surrounding objects is on the same 

 scale, as regards contrast of light and shade, in moonlight as in sun- 

 light — that is to say, in each case the source of light is an object sub- 

 tending an angle of about 30' at a distance which is practically 

 infinite. Whatever difference may seem to exist must be of physio- 

 logical or psychical origin. The deeper shadows in moonlight probably 

 afford too little stimulus to fully excite the sensation of vision even in 

 an eye accustomed to darkness ; but it must not be forgotten that we 

 accentuate this difference by a habit of looking at the moon itself and 

 at the bright sky near it, thus blinding ourselves to the faintly illumi- 

 nated details of the shadows. If we were to do the same with sunlight 

 the shadows would seem equally lacking in detail. In a room arti- 

 ficially lighted there is seldom so much contrast between lights and 

 shadows. Light-coloured objects are usually to be found in close 

 proximity to the lamps, even where white shades or globes are not used 

 to diffuse the light. It is less easy to demonstrate the phenomena of 

 temporary colour-blindness under these circumstances, owing to the 

 greater relative intensity of the dazzle-tints'^ resulting from the action 

 of the diffused light before the experiment began. Until these are gone 

 the retinal fatigue is not confined to one colour. If in experimenting 

 with moonlight the observer accidentally looks at the moon's disc 

 before his eye is protected by the coloured screen, a well-defined after- 

 image is produced, and the subsequent phenomena of colour-blindness 

 are only locally modified, whereas if an after-effect even of less 

 intensity, due to diffused light, is present, the colour-blindness may be 

 to a great extent masked. 



The other point of interest in connection with this experiment is 

 that colour-blindness has been produced by light no stronger than that 

 reflected by ordinary pigments in sunshine. That this is so is evident 

 if we look at the moon's disc in the daytime through the same red 

 glass and lens and compare it with a piece of coloured paper. It can 

 therefore be hardly maintained that the condition of temporary colour- 

 blindness should be regarded as a pathological result of excessive 

 stimulation of the colour sensations. Merely to look for a few seconds 



* It will be convenient, in describing my own experiments, to retain this word, 

 wbich I bave used to signify tbe " elementary component sensations of the 

 positive after-efPect." ' Pbil. Trans.,' B, vol. 191 (1899), p. 6. 



