of Colour-disease. 9 1 



1. Colour-blind persons with two intervals in the grating-spec- 

 trum may in fact be blind to red, and indeed blind to potassium- 

 red. Mr. F* was tried, a Daltonist, who in August 1860 saw 

 two intervals. 



He saw the red lithium -line (at 13*7), the blue potassium- 

 line (at 2*5 of the photographed scale), but not the red potas- 

 sium-line (at 15) ; even when its position on the scale was 

 pointed out, it was impossible for him to make it out. 



With the gas- spectrum a small shortening was also seen. 



2. Colour-blindness may extend over half the spectrum. On 

 one occasion Mr. Bf was tried, a Daltonist, who five years ago 

 saw three spaces. He no more saw the red potassium-line than 

 didF %, not even when the place was indicated; the blue very well. 



He saw the chloride-of-strontium spectrum as we did (between 

 13 and 14), at any rate when his attention was directed to it. 



The spectrum of the gas-lamp ended at 2*9 in all our cases. 



Mr. M was then tested §, who varied at that time in his state- 

 ments, on different days saw between four and five intervals, and 

 in other respects showed great variation in his perception of 

 colour, now only saw three spaces. He saw the red potassium- 

 line, and the red chloride-of-strontium immediately, but the 

 violet line not even when its place on the scale was indicated. 



A good gas-lamp spectrum ended in his case exactly at 6 ; with 

 myself, M. Mitscherlich, and Mr. B at 2*9. A second inves- 

 tigation some days later gave the same result. 



Mr. L was lastly tested ||, who then saw six intervals. 



A good gas-spectrum, which, as has been stated, began at 2*9 

 and extended to 15*5, ended in his case on the one side at between 

 5 and 6, nearer 5 ; on the other at about 13. 



In the course of the observation he made the characteristic 

 remark that from 3 to 1 the field of view was brighter than 

 from 3 to 5, yet by no means so bright as between 5 and 7 ; 

 as might be expected, because the field of view is somewhat 

 brightened by the flame for the photographic scale. He further 

 stated that he could see the figures of the scale 14-18 very di- 

 stinctly, but scarcely 12 and 13 ; while I and others could per- 

 ceive only an indication of 14, because our gas-spectrum extended 

 beyond this figure. 



Of the potassium-spectrum he saw neither the red nor the 

 blue line, not even when the position was shown (15 and 2'5). 

 Of the chloride-of-strontium spectrum he saw only the first pale 

 orange bands at 12*4; not one of the five red ones, which lie 

 between 127 and 14. 



* v. Gvafe's Archiv, vol. vii. p. 87- t Ibid. p. 80 . 



% He was not present. § Grafe's Archiv, vol. vii. p. 104. 



|| Ibid. p. 99. 



