240 ON ACCIDENTAL OR SUBJECTIVE COLORS. 



illuminate it in a uniform manner and throw a liglit on the second mirror equal to 

 that of the first. An opaque screen served to separate the candles and the flame of 

 the alcohol in order that each of the mirrors should receive but one of the two 

 lights. I had thus two luminous fields of the same brightness, one of which, 

 however, was illuminated by a homogeneous yellow light, and the other by a 

 light which, without being white as that of day, was still composite enough for 

 the case in question. 



"I then placed before these two luminous fields identically similar apparatus 

 of irradiation, so that, by observing them simultaneously, it was easy to see 

 whether the irradiations developed by the two lights sensibly differed. Now, 

 this comparison, made by the two persons above named and myself, showed us no 

 appreciable difference; each apparatus manifested a decided irradiation, but that 

 which proceeded from the compound light had neither more nor less extent than 

 that produced by the homogeneous light. 



" These facts seem to me to lead unavoidably to these conclusions : that if the 

 aberration of refrangibility in the eye must needs be admitted, irradiation should 

 be attributed to some other cause, and that the effect of aberration must be con- 

 sidered as entirely masked, under ordinary circumstances, by the band of 

 irradiation." 



This vindication by M. Plateau has remained unanswered. 



Colors produced in the eye by pressure or other indirect causes. — It is not 

 light alone which produces in our eye a luminous sensation. It is well known 

 thiit irritation, of whatever nature, which affects the retina occasions an impres- 

 sion of light. Compress, for instance, the jugular veins, and, however profound 

 the surrounding darkness, flashes and luminous rays will seem to glance before 

 the eyes. Under numerous and common circumstances the most varied colors 

 appear to us without the agency of external light. The chief accidental causes 

 of the production of these colors are : 



1. Mechanical irritation or pressure. — A slight compression applied to the 

 external angle of one eye produces a yellow spot in the same angle of the other 

 eye if it is closed, and a blue spot if it is open ; every one can easily make ex- 

 periment of this. Newton had remarked that a weak pressure with the finger 

 on the globe of the eye or blow on that organ produced the sensation of color ; 

 these colors he thought vanished instantly when the finger remained unmoved, 

 but Brewster has proved that they continue as long as the pressure is maintained. 

 He has shown also that when the eye is pressed so as to compi-ess lightly the 

 pulpy substance of the retina a circular spot of colorless light is produced, al 

 though the eye be in complete darkness and has not for hours been exposed to 

 the light. If the eye is then illuminated it will be found that the compressed 

 part of the retina is more sensible to the light than any other part, and seems in 

 consequence more luminous. A slight compression of the retina, therefore, aug- 

 ments the sensibility for the light it receives, and gives rise to a sensation of 

 light. On the contrary, adds M. Brewster, when the retina is dilated under the 

 influence of light, it experiences an absolute blindness or becomes insensible to 

 luminous impressions. These properties of the retina often evince themselves 

 spontaneously, with different modifications, in consequence of the movement of 

 the eye by its own muscles. Pressure, when it is too feeble to produce a lumin- 

 ous impression, may modify other impressions first produced on the retina. If, 

 after having been directed toward the sun, the eye sees a reddish brown spec- 

 trum, pressure on another part of the retina will cause a change of the spectrum 

 to green, and the brown will reappear when the pressure has ceased. 



When pressure is applied at the same time to both eyes, the luminous appear- 

 ances become remarkable in another way, and particularly in then affecting a 

 regular form, which seems to be the same in all persons. If, for example, pres- 

 sure be exerted on both eyes in opposite directions, whether to separate them or 

 bring them closer together, there will first be perceived a bluish red light; then, 



