Under the condition of full, normal daylight illumination a surface which reflects a fair proportion of ultra-violet, as well as the visible spectrum, is neutral or white in the vision of the bee. If the percentage of reflexion of 3600 A. is less than one-third, or about one-quarter, of the percentage of reflexion in the visible light, a degree of coloration is obtained which is sufficient to be noticed by the bees in training experiments.
When both are placed under ultra-violet absorbing filters, white paper in the vision of the bee most closely resembles a blue-green paper that possesses the highest amount of reflexion at 4900 A. Bees which have been trained to visit a blue-green surface covered by a filter glass do not do so any longer, if, by removing the filter, ultra-violet light is added to the reflexion. The paper that possesses now two peaks of maximum reflexion, one at 4900 and the other at 3600 A., appears a light grey to the bee. On the other hand, when bees have been trained to visit an ultra-violet surface--white paper under a filter that absorbs the visible light completely--do not do so any longer if by lifting the filter the whole range of visible light is added to the reflexion of ultra-violet. The white paper at once becomes unattractive to the bee.
There remains no doubt, that among the four qualities of colour discriminated by the bee (see Fig. 1) the first and the third on one side and the second and fourth on the other are complementary colours for this insect and presumably for many others.
In the European flowers visited by bees three principal colours are now finally known: (1) that colour which is perceived if the main bulk of light reflected by the petals lies between 6500 and 4900 A. (orange-yellow for bee and man), (2) that which is perceived if the reflexion extends from 4900 to 4000 or 3500 A. (blue-violet for bee and man), and (3) that which is perceived if the reflexion extends from 6500 A., or nearer to the red end of the visible spectrum, to 4000 A. (blue-green for the bee, but white, pink, light purple, bluish or yellowish for man).
Ultra-violet seems never to occur in European flowers in such a way as to make petals which are brilliantly white for man to appear equally white for the bee.