216 



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



of the spectral flash by the white flash, a short interval of darkness 

 between them being preferable to the smallest overlap. For this 

 reason the shaft is fitted with a screw nut, which being slackened, the 

 angular position of the slits with respect to the sectors can be accu- 

 rately adjusted. On rotating the discs steadily, but not too quickly, a 

 spectrum of complementary colours is seen with the greatest distinct- 

 ness. By placing a narrow strip of black card across the mouth of 

 the scale tube, a portion of the white flash may be stopped out, allow- 

 ing the normal spectrum to be seen in that part of the field. It is 

 necessary, however, to shade the corresponding part of the slit some- 

 what, so that the normal spectrum may not overpower the comple- 

 mentary spectrum. The colours as I see them are as follows : — Red is 

 replaced by Prussian blue, green by purple (a red shade of Hoffinann's 

 violet), blue by orange, and violet by yellow. To show the comple- 

 mentary of violet it is necessary to use sunlight, or, better still, the 

 arc light. I have never been able to see it properly by any of the 

 methods involving the use of white card or paper surfaces as reflectors. 



The experiments of Section 3, for which a wide dispersion was 

 required, were made with a large direct-vision spectroscope belonging 

 to the Marlborough Collection, for the use of which I am indebted to 

 the Aldrichian Demonstrator of Chemistry, Mr. W. W. Fisher. I have 

 also to thank Professor Gotch for the use of the electric light in the 

 physiological laboratory. The remainder of the work was done at 

 Reading College, and the expenses have been defrayed by a portion 

 of the sum of £10 allotted to me by the Royal Society out of the 

 Government Grant. 



" On the Production of Artificial Colour-blindness by Moonlight." 

 By George J, Burch, M.A. Oxon., Reading College, Reading. 

 Communicated by Professor GoTCH, F.R.S. Received January 

 30,-— Read February 8, 1900. 



Since the publication of my paper on "Artificial Colour-blindness"* 

 I have found a very general and not unnatural tendency to regard the 

 results described therein as phenomena of a pathological condition 

 induced by the severe strain to which the structures of the eye had 

 been subjected. In my paper I indicated, perhaps too briefly, that 

 this could not be the case, since " the same general phenomena are 

 observable alike with strong sunlight and with the faintest light the 

 eye is capable of perceiving." 



The purpose therefore of the present communication is to describe 

 some of the experiments on which that statement was based. 



* ' Phil. Trans.,' B, vol. 191 (1899), p. 1. 



