ON ACCIDENTAL OR SUBJECTIVE COLORS. 245 



as before, but in a brighter manner, and the spaces between the ramifications 

 exhibit an innumerable quantity of small vessels, which anastomose in different 

 directions, and which were invisible in the preceding experiment. At the very 

 centre of the field of vision there is a small circular space in which no trace of 

 vessels is seen. 



M. Melloni on the coloring of the retina and the crystalline. — According to 

 the theory which we have long developed and defended, vision is the result of 

 extremely rapid vibrations excited in the retina by a certain series of ethereal 

 undulations. The vibrations will depend less on tlie quantity of movement 

 than on the particular disposition of the retina which renders it apt to vibrate 

 with more or less facility, in unison with such or such an ethereal vibration; 

 this, in terms of acoustics, would be a sort of resonance, proceeding from the 

 accord or harmonic relation established between the tension of the molecular 

 groups of the retina and the period of undulation of the incident ray. Those 

 undulations, which are between the yellow and the orange, which correspond, as 

 Fraunhofer has proved, to the maximum of luminous intensity, will better har- 

 monize witb the constitution of the retina, and will communicate to the molecules 

 a more intense vibratory movement. The quantity of light perceived would de- 

 pend on the intensity of the molecular vibrations, and the color on the number 

 of the vibrations. By assuming that the red and violet undulations find in the 

 retina a less perfect consonance than the yellow undulations, it will be readily 

 comprehended that the former give a less quantity of lig'nt. This hypothesis 

 is the more plausible, inasmuch as, followed to its final consequences, it leads to 

 a very happy explanation of the obscure radiations, chemical or calorific, situ- 

 ated outside of the solar spectrum, radiations endowed, as will be shown farther 

 on, with all the properties of luminous rays, visibility excepted. 



It must needs, then, be admitted that the ethereal undulations of different 

 colored bands of the spectrum have a different aptitude for exciting the vibra- 

 tions of the retina, and that the maximum of effect pertains to the yellow. 



It has been shown that the substances which vibrate with the same fiicility 

 under the action of luminous undulations of any length whatever are white ; 

 colored substances, on the contrary, are those which vibrate with more intensity 

 under the influence of one or of several species of luminous undulations, while 

 showing themselves less sensible to others. Now, it is a constant fact that the 

 yellow undulations produce by consonance the maximum of effect on the retina. 

 The retina, then, must be yellow, and not colorless, as has been thought till 

 now. This conclusion rests on a perfect analogy of luminous properties be- 

 tween the retina and the different bodies of nature. It may be conceived, how- 

 ever, that the vital force might communicate to the retina, even if white, a degree 

 of excitability more in unison with the color of yellow. To decide, therefore, 

 this question of the color of the retina, appeal must of necessity be had to ob- 

 servation, and this has been done by M. Melloni. 



We must presume, he says, that no observer sufliciently experienced has 

 examined with attention this precious membrane; for, otherwise, he would cer- 

 tainly have recognized that it possesses a yellow tint very distinguishable. If 

 we attentively observe a section of the retina, it will be seen that its thickness 

 increases from the borders to the centre, occupied by the yellow spot of Sum- 

 mering, or rather of Buzzi, an Italian physician, who discovered it in 1782. 

 Nor is it the central and thickest part of the retina alone which is and which 

 exhibits a yellow tint to' the eye, whether naked or equipped with a microscope ; 

 it is the entire retina, and if near the border the color is no longer distinct, it is 

 solely on account of the exces.<ive thinness of the membrane. The yellow color, 

 indeed, cannot be mistaken when the retina is observed under an oblique inci- 

 dence, or when the thin portions are folded upon themselves. In the eye of a 

 man dead with the symptoms of very intense jaundice, the whole retina had a 



