86 Dr. E. Rose on the Doctrine 



It has, secondly, been found, with the same invariability, that 

 as the disease increases, the patient ceases to perceive only that 

 light which had previously the greatest or smallest refrangibi- 

 lity among his visible rays. 



Colour-blindness is always characterized by a shortening of 

 the spectrum, and never by an interruption*. A complete and 

 accurately defined spectrum thus forms by its extent a measure of 

 the degree of colour-blindness. 



The first and most pronounced case of colour-blindness affected 

 myself in August 1858. A brilliant spectrum, the most beau- 

 tiful I have seen, was used as a measure. My late friend, Dr. 

 Otto Hagen, used it in his beautiful investigations on the absorp- 

 tion of light in crystals, the results of which are partially given 

 in vol. cvi. of PoggendorfPs Annalen. The spectrum, as repre- 

 sented theein, fig. 2, plate 3, was produced by reflecting a solar ray 

 from the silvered mirror of a heliostat, through two slits placed a 

 metre apart, upon a flint-glass prism placed immediately behind, 

 by which it was decomposed in a dark chamber. At a distance 

 of eight metres its constituents were observed by the tele- 

 scope of a theodolite with cross threads. By a micrometer- 

 arrangement the prism can be turned at will, and the red or the 

 violet end introduced. The observations were made on bright 

 summer days, and only a transient little cloud obscured the sun. 

 Apart from such interruptions the spectrum was seen in its 

 entire splendour, interrupted by about thirteen of Fraunhofer's 

 lines, terminating sharply on both sides on the black back- 

 ground of the telescope, and so long that in the field of view the 

 entire red could not be seen at once. Its length was sometimes 

 momentarily altered at the red end ; and on different days its 

 length was different, without its being always possible to detect 

 with the naked eye the fine haze which covered the sun, and 

 which then sometimes agglomerated into clouds. The result 

 was tested and repeated on different days, but we always agreed 

 in the position of the cross threads at the ends of the spectra. 



The result was different when I had taken ten grains of san- 

 tonic acid. Two hours afterwards my spectrum ended at the line 

 B, while Hagen placed the cross further, where a must have been, 

 in a space which was perfectly dark to me. It remained thus 

 for some time ; though once for about half an hour he observed 

 the spectrum to end at B, while at the same time I found it to 

 end at C. On the next day the cross was placed by both in the 

 same position. 



It follows from this that yellow vision, for only this occurredf, 



* "On the Action of the essential Constituents of Santonicum," in Vir- 

 chow's Archiv, 1859, vol. xvi. pp. 223-253 ; and vol. xviii. pp. 15-43. 

 f Case 12 inVirchovv's Archiv, vol. xvi. p. 239 ; vol. xviii. p. 18. 



