On artificial temporary Colour-blindness. 35 



The observed rotation in oxygen gives a value for this of 

 3'5 x 10~ u , so that it is about equal to the calculated value. 



The discrepancy may be due to the fact that p is not the 

 same for all spectral lines, and we could not reasonably expect to 

 get an accurate result by assuming that the Zeeman effect in zinc 

 can be nsed in calculating the Faraday effect in oxygen. On the 

 whole I think the calculation shows that what must be a vera causa 

 for a Faraday effect is the whole cause of it. From the mere fact 

 that rotatory pola.risation is approximately inversely as the square of 

 the wave-length and consequently vanishes for long waves, it follows 

 that it is essentially a dispersion phenomenon. 



" On artificial temporary Colour-blindness, with an Examination 

 of the Colour Sensations of 109 Persons." By GEORGE J. 

 BURGH, M.A. Communicated by Professor GOTCH, F.R.S. 

 Received February 5, Read February 17, 1898. 



(Abstract.) 



By exposing the eye for a sufficient length of time to bright 

 sunlight in the focus of a burning glass behind suitably chosen 

 transparent screens, it is possible to induce over the whole retina a 

 condition of temporary colour-blindness. 



After red light the observer is for some minutes completely red- 

 blind, so that scarlet geraniums appear black and roses blue, while 

 yellow flowers seem various shades of green, and purple flowers look 

 violet. The same mistakes are made in sorting Holmgren's wools as 

 by the red-blind. 



Temporary violet blindness may be brought about by using a tank 

 of ammonio-sulphate of copper. While it lasts, violet wools look 

 black and purple flowers crimson, but the green foliage appears of a 

 richer tint than usual. The recovery from violet blindness is very 

 slow. 



Green blindness may be brought about by exposing the eye to 

 light through three thicknesses of green glass. The colour-scheme 

 of the landscape during this condition is that of a picture painted 

 with vermilion, flake-white, and ultramarine, variously blended. 



Purple blindness may be produced by a combination of films 

 stained with magenta and aniline violet, by which the green is 

 absorbed. During purple blindness the vision is practically mono- 

 chromatic, no colour being visible but green. 



If one eye is rendered purple-blind and the other green-blind, the 

 observer sees all objects in their natural colours but with a curiously 

 exaggerated perspective due to the difficulty of combining the 

 images perceived by one eye with those visible to the other. 



