Hastings — Optical Errors of the Human Eye. 407 



III. Phenomenon of "Fluttering Hearts" 



This phenomenon, upon which much has been written, is 

 thus described by Helmholtz :* — " A peculiar phenomenon, 

 which perhaps belongs in the same domain as that of the flick- 

 ering rotating disks, is that of the so-called fluttering hearts. 

 On a colored sheet of stiff paper are placed figures of another 

 vivid color ; red and blue seem to yield the best results — the 

 colors must be very vivid and saturated. If one looks at the 

 sheet while it is moved to and fro with a certain quickness, 

 the figures seem to move themselves and to shift forwards and 

 backwards on their support. The cause of this appearance 

 seems to depend upon the fact that the visual impression for 

 different colors does not originate and die out with equal 

 quickness, and consequently the blue appears left somewhat 

 behind the red in the path described by the sheet." 



Numerous experiments with a considerable number of 

 observers made in accordance with this description proved 

 wholly futile. Small disks of vividly colored paper scattered 

 upon strongly contrasting grounds were tried under greatly 

 varying circumstances of illumination and of observer with- 

 out once succeeding. It was only after one of my colleagues 

 brought me a particular book having green and black lettering 

 printed upon a red cover, both colors being nearly saturated 

 and of approximately equal luminosity, that the real phenome- 

 non could be observed by me and exhibited to others. Even 

 in this case it could not be recognized in a good illumination, 

 either by day light or by artificial light, when the lighting was 

 such that the texture of the surface and outlines of the green 

 letters was well seen ; but when illuminated by a single source 

 of light, sufficiently remote so that the sharpness of the green 

 letters was lost, the effect became absolutely startling — the 

 green letters appearing to slip about among the black in a most 

 unaccountable way. The astonishment shown by all to whom 

 I have exhibited this optical illusion is a sufficient proof of its 

 rarity in ordinary experience. 



The indicated conditions of success in the experiment seem 

 to be these : — 



(a) The saturated colors must differ widely in refrangibility 

 and not too widely in luminosity. ' 



(b) The position of one of the colors in the visual field 

 must be well determined — as by the sharply printed black let- 

 ters in the object described — while the other must have its 

 outlines ill defined. 



With attention to these precepts I have been able to secure 

 the illusion invariably with every one with whom I have experi- 

 * Helmholtz, Physiol. Opt., 2te Aufl. p. 533. 



