OTHER VALUES OF YELLOW AND RED FILTERS 197 



Red Filters and the Rayleigh Effect — A very widespread supersti- 

 tion, showing itself in such things as red airport beacons and amber fog- 

 lights on automobiles, is the notion that some colors — notably red — pen- 

 etrate fog better than do other colors or white light. The supposed phe- 

 nomenon is attributed to the 'Rayleigh effect', which is the scattering of 

 light inversely as the fourth power of the wavelength. The short waves 

 are scattered the most, the red and infra-red ones scarcely at all, resulting 

 in the blue coloration of the sky and in the remarkable clear pictures 

 which can be taken through haze with infra-red-sensitive plates. 



But as far as the visible spectrum is concerned, there is no Rayleigh 

 scattering at all when the particles which cause the scattering are larger 

 in diameter than 0.75 [X. Natural mist and fog droplets, and solid particles 

 suspended in natural waters, are invariably at least several times this size, 

 and scatter light quite irrespective of wavelength. Red oil-droplets can- 

 not, then, be designed to sharpen images by eliminating Rayleigh-scat- 

 tered light in misty weather or in water, as some have thought. The tur- 

 tles and birds have nothing in common, and if this inclusive explanation 

 will not hold for the red oil-droplets of the two groups, room is left for 

 independent explanations of the two cases. 



Value of Red Oil-Droplets in Birds — Most birds are such early ris- 

 ers that they expose themselves to Rayleigh scattering — not by gross mist 

 particles, but by molecules of water and gases in the atmosphere of even 

 the clearest of sunrises. At this time of day the sun's rays slant through 

 such a long atmospheric pathway that they appear reddened, the same 

 being true of the sunset — which is more familiar to most of us. The bird, 

 getting in most of his day's work at dawn and shortly after, is aided by 

 his red droplets. As the day wears on and the sunlight whitens, the yellow 

 (and on dull days, the colorless) droplets take over — the orange ones 

 affording a smooth transition. If this explanation is true, one would ex- 

 pect late-rising birds to have few red droplets. This is indeed the case, for 

 whereas the song-birds average 20% red droplets, the hawks have but half 

 of this number; and in the crepuscular swifts and swallows there are but 

 3% to 5% red droplets. 



Value of Red Oil-Droplets in Turtles — The significance of the red 

 droplets of turtles is rather different. More than any other diurnal verte- 

 brates, they have the problem of seeing over the glary surface of water. 

 Since they have intensity to spare, they can afford red droplets for the 

 even greater effect upon chromatic aberration which a red filter will have, 



