54 ON MAGNITUDE [ch. 



of small and optically isolated simple eyes into a compound eye, 

 and in the insect Nature adopts this alternative possibiHty*. 



Our range of vision is limited to a bare octave of "luminous" 

 waves, which is a considerable part of the whole range of Hght-heat 

 rays emitted by the sun; the sun's rays extend into the ultra-violet 

 for another half-octave or more, but the rays to which our eyes are 

 sensitive are just those which pass with the least absorption through 

 a watery medium. Some ancient vertebrate may have learned to 

 see in an ocean which let a certain part of the sun's whole radiation 

 through, which part is our part still ; or perhaps the watery media 

 of the eye itself account sufficiently for the selective filtration. In 

 either case, the dimensions of the retinal elements are so closely 

 related to the wave-lengths of light (or to their interference patterns) 

 that we have good reason to look upon the retina as perfect of its 

 kind, within the hmits which the properties of hght itself impose; 

 and this perfection is further illustrated by the fact that a few 

 light-quanta, perhaps a single one, suffice to produce a sensation "j". 

 The hard eyes of insects are sensitive over a wider range. The bee 

 has two visual optima, one coincident with our own, the other and 

 principal one high up in the ultra-violet J. And with the latt^er the 

 bee is able to see that ultra-violet which is so well reflected by many 

 flowers that flower-photographs have been taken through a filter 

 which passes these but transmits no other rays§. 



When we talk of hght, and of magnitudes whose order is that of 

 a wave-length of hght, the subtle phenomenon of colour is near at 

 hand. The hues of hving things are due to sundry causes; where 

 they come from chemical pigmentation they are outside our theme, 

 but oftentimes there is no pigment at all, save perhaps as a screen 

 or background, and the tints are those proper to a scale of wave- 

 lengths or range of magnitude. In birds these "optical colours" 

 are of two chief kinds. One kind include certain vivid blues, the 



* Cf. C. J. van der Horst, The optics of the insect eye, Acta Zoolog. 1933, 

 p. 108. 



t Cf. Niels Bohr, in Nature, April 1, 1933, p. 457. Also J. Joly, Proc. R.S. (B), 

 xcn, p. 222, 1921. 



f L. M. Bertholf, Reactions of the honey-bee to light, Journ. of Agric. Res. 

 XLm, p. 379; xliv, p. 763, 1931. 



§ A. Kuhn, Ueber den Farbensinn der Bienen, Ztschr. d. vergl. Physiol, v, 

 pp. 762-800, 1927; cf. F. K. Richtmeyer, Reflection of ultra-violet by flowers, 

 J num. Optical Soc. Amer. vii, pp. 151-168, 1923; etc. 



