246 ON ACCIDENTAL OE SUBJECTIVE COLOES. 



most decided tint, and the central spot had acquired an extraordinary vivacity of 

 coloration. 



The yellow of the central spot, which in our view constitutes the natural 

 color of the retina, fades and disappears as age advances — an observation which 

 M. Melloni has not found reported in any treatise on physics. It presents itself, 

 however, in a very marked manner on a comparison of retinas taken at different 

 epochs of life. From the change of color in the retina there would necessarily 

 result an alteration in regard to the perception of the elementary rays ; but, by 

 one of those innumerable provisio'ns which surprise us at every step in the 

 science of the development of organized beings, nature has taken measures to 

 repair the effects of this disorder. The crystalline is perfectly limpid and color- 

 less up to the age of twenty-five or thirty years ; but this period having passed, 

 we see it assume a very slight tint of straw-colored yellow, which is developed 

 at first on the central part, afterwards attains the borders, augments progressively 

 in value, and finally becomes, in aged persons of seventy-five or eighty years, 

 quite as decided as the color of yellow amber. If we consider the effect pro- 

 duced upon the sight by this new development of color, it will be at once com- 

 prehended that the yellow acquired by the crystalline is destined to repair the 

 loss of that tint in the retina. To show that the sum of the two variations 

 really compensate for one another, M. Melloni procured a number of eyes of very 

 different ages, and, having extracted the crystallines, he placed the latter on the 

 central parts of the corresponding retinas; all the systems were thus found to 

 present the same shade of yellow. . The experiment carried to the two opposite 

 limits is one of great interest; for in early youth the coloration, which is not 

 yet developed in the crystalline, is amply manifested on the retina, while in de- 

 crepitude it has entirely retreated to the crystalline, leaving no trace of itself 

 on the retina. The aged crystalline placed beside the young retina presents the 

 same tint, notwithstanding the vast difference of constitution. 



The appearance and progress of the yellow tint in the crystalline constitutes, 

 therefore, a true process of what might be called attuning, put in operation by 

 nature for the purpose of maintaining at the same luminous pitch the instrument 

 of vision. It can thus be understood why white continues to be white for our 

 eyes at all the epochs of life, notwithstanding the increasing coloration of the 

 crystalline and the interposition of a yellow medium . between external objects 

 and the retina. 



DALTONISM, OR INNATE IMPERFECTIONS IN THE PERCEPTION OF COLORS. 



Definition. — There are persons whose eyes are incapable of distinguishing 

 colors. They readily perceive the form of objects, distinguish light from dark- 

 ness, appreciating the slightest differences ; but they mistake certain colors. Their 

 retina, insensible for some tints, cannot transmit their impression to fEe brain. 

 The number of such persons is much more considerable than is generally thought. 

 Of forty youths who pursued the higher studies of the gymnasium at Dresden, 

 M. Seebeck found five to be daltonians, some of whom, however, were without 

 consciousness ot their infirmity. One of the singular features of the defect is 

 that it is often hereditary. Three remarkable memoirs — published, one, in 1837, 

 by M. Seebeck, jr.; the second, at Geneva, by M. Elie Wartman; the third, at 

 Paris, by M. Victor Zokalski, in 1841 — seem to embrace all that relates to this 

 difficult question, and to the analysis of these we propose to devote a few pages. 



Observations. — As all researches on the subject set out with the observations 

 made by Dalton on himself, an account of which he inserted in the memoirs of 

 the Literary Society of Manchester, we shall give substantially a transcription 

 of it: "I had always thought," says Dalton, "that their true name was often 

 not applied to colors. Violet, to designate the flower of that color, seemed, of 

 course, proper ; but if the term red were substituted, this license appeared to 



