1845] WRITINGS OF JOSEPH HENRY. 239 



Dr. Seebeck, as well as Professor Wartmann, has made a 

 series of experiments to determine whether a person of this 

 peculiarity of vision possesses the power of perceiving differ- 

 ences in colors which appear identical to us. The result of 

 the investigations of both these philosophers was that he 

 does not. Another problem has also been solved by the last- 

 mentioned gentleman, in reference to the difference between 

 a person with this defective vision and one of ordinary con- 

 ditioned sight, in the perception of complementary colors. 

 He found that colors which we regard as complementary, or 

 such as when mingled together produce white, do not appear 

 as such to those affected with this abnormal vision. They are 

 not however insensible to accidental colors, but the feeling: 

 which results from the fatigue of attempting to produce 

 these appears to be more painful in them than in us. 



Various hypotheses have been advanced by different per- 

 sons for the explanation of color-blindness. Mr. Dalton 

 supposed that his peculiarity of vision, as well as that of 

 those whom he had examined, depended on the fact that the 

 vitreous or principal humour of the eye, in these cases, instead 

 of being colorless and transparent was tinged with a blue. 

 After his death, in obedience to his own instruction, his eyes 

 were examined by his medical attendant, Mr. Eansome, but 

 the vitreous humour was not found to exhibit any tinge of 

 blue; on the contrary, it was of a pale yellow color. Objects 

 viewed through it were not changed in color as they should 

 have been had the hypothesis been true. Indeed, were the 

 supposition correct, the same effect should be produced by 

 blue spectacles, which is known not to be the case. 



Stewart, Herschel, and others are of the opinion that this 

 malady of vision is attributable to a defect in the sensorium 

 itself, which renders it incapable of appreciating the differ- 

 ences between the rays on which the sensation of color 

 depends. Sir David Brewster conceives that the eye, in the 

 case of color-blindness, is insensible to the colors at one end 

 of the spectrum, just as the ear of certain persons is insen- 

 sible to sounds at one extremity of the scale of musical notes, 

 while it is perfectly sensible to all other sounds. He knows 



