232 ON ACCIDENTAL OR SUBJECTIVE COLORS. 



iudigo grave. Nobili undertakes to explain this last difference by an original 

 comparison between colors and sounds. An exclamation, a cry of joy, are com- 

 posed of notes which proceed from grave to shrill ; a wail, an accent of pain, 

 descend from shrill to grave ; the same series of notes chanted by ascending from 

 grave to shrill, or descending from shrill to grave, produces contrary effects ; 

 gaiety in the first case, sadness in the second. Now, if we take the comple- 

 mentary tints of red and violet, which are the bluish green and the greenish 

 yellow, it will be seen that we pass, in the first case, from longer vibrations to 

 shorter ones ; in the second case it is the reverse. This digression will be par- 

 doned us, if only as a grateful recollection of the eminent and excellent savant 

 who honored us with a share of his friendship. 



Singular effect in optics. — Before proceeding to treat of irradiation, we must 

 notice a singular effect produced by the juxtaposition of certain colors which was 

 observed by M. Wheatstone. This savant had remarked on some occasion that 

 a carpet having a small green and red figure presented, when it was illuminated 

 by gas-light and attentively observed, the appearance of a movement in all the 

 figures of the design. To this optical illusion the name of palpitating hearts 

 was given, because one of the figures of the pattern was a heart. M. Wheatstone 

 made a trial of several other designs formed of other pairs of complementary 

 colors, and ascertained that the movement Avas perceptible, but never so decidedly 

 as with the colors green and red. He was not able to distinguish the phenom- 

 enon except by the light of gas ; but M. Brewster afterwards perceived that it 

 was produced also by the solar light admitted through a small hole in the dark- 

 ened chamber. In order to explain this fact, M. Brewster recalls the experiment 

 which shows that any fixed object appears to move when the light that illumi- 

 nates it is constantly changing in position and intensity. The experiment is 

 made by moving a candle before a statue rapidly and in all directions; the 

 flashes of light and the shadows, incessantly shifting, produce different appear- 

 ances which resemble the movements of the figures on the red and green carpet. 

 In the case of the palpitating hearts, the mixture of two complementary colors, 

 whether it be regarded directly or as producing accidental impressions, forms a 

 succession of shades and lights which give rise to the apparent movement of the 

 figures. When one of the Jiearts is looked at fixedly, it ceases to move almost 

 entirely, while the others, which are seen obliquely, appear to vibrate in a very 

 distinct manner ,• this effect is attributable to the augmentation of the sensibiHty 

 of the retina in oblique vision, in proportion as the point which receives the 

 impression is more remote from the opening of the pupil. 



IRRADIATION. 



Definition. — Irradiation is properly the phenomenon by virtue of which a 

 luniiuous object environed by an obscure space appears more or less amplified. 

 This apparent encroachment of the border of a luminous object on the obscure 

 space which surrounds it, involves an opposite illusion in "the case of a dark 

 object projected on a luminous field : the dimensions of this object appear di- 

 minished, because the irradiation produced along its outline by the luminous 

 field falls within the outline. A singular difference of opinion divided astrono- 

 mers in relation even to the existence of aberration ; some admitting it, others 

 calling it in doubt ; these doubts solely resulted from the fact that astronomers 

 had not clearly distinguished the part played by ocular irradiation in observa- 

 tions made by means of the telescope. 



Existence of irradiation. — To verify the phenomenon it suffices to cast the 

 eyes on the luminous crescent of the moon. Every one knows that when the 

 moon appears under the form of a crescent, while at the same time the rest of 

 its ditrk IS seen faintly illuminated by an ash-colored light, the exterior outline 

 of the luminous portion seems to protrude considerably beyond that of the ob- 



