Vision. 249 



Daltonism, was twenty-five years of age before lie was dis- 

 tinctly convinced of his peculiarity of vision. Yet so great 

 was this peculiarity that in describing it he wrote : " Crimson 

 appears a muddy bkie by day, and crimson woollen yarn is 

 much the same as dark blue;" and further recorded that the 

 one side of a laurel leaf seemed to him a good match to a 

 stick of sealing-wax, and the other side to a red wafer. After 

 Dalton had attracted attention to the matter, it was found 

 that his case was so far from being solitary that colour-blind- 

 ness in different degrees was not unfrequently to be met with. 

 This being the case, it is evident that, on railways and at sea. 

 danger signals dependent on the number or position of lights 

 are preferable to those dependent on colour, and that red 

 lights are specially objectionable. 



185. The laws which regulate the colours of the ocular spec- 

 tra, above alluded to, are curious, and not so easily explained 

 as we are often asked to believe. If a brightly-coloured object 

 on a white ground be steadily gazed at, on looking away from 

 it to the surface on which it lies, or, still better, looking tc a 

 dark surface, or shutting the eyes, the image of the object 

 remains before the sight, in the complementary colour that 

 is to say, in the colour which, added to that of the object 

 gazed on, would make white light. If the object be red ; the 

 spectrum will be green, and if the object be blue, there will 

 be an orange spectrum ; and the explanation commonly given 

 is, that the part of the retina on which the coloured rays have 

 fallen becomes by exhaustion less affected by the rays of the 

 same colour in the light around, while it is affected by all the 

 colours entering into white light. The following circum- 

 stances, however, show that explanation to be insufficient : 

 1 . The brightest complementary-coloured spectrum is ob tained 

 by shutting the eyes, or looking into total darkness. 2. 

 While continuing to gaze on the coloured object, a ring of 

 light more brilliant than the surrounding surface makes its 

 appearance round about, and the spectrum seen on the 

 white surface is of the same shade as that ring of light. 3. 

 If the object gazed at be on a dark ground, the ring about 

 it will be, not one of light, but of greater darkness; and 

 the spectrum, if cast on that ground, will be of the same 

 shade. 4. If a very brilliant spectrum be obtained, its colour 



