Dichroism

Abstract

"Will all great Neptune's ocean wash this blood Clean from my hand? No, this my hand will rather The multitudinous seas incarnadine, Making the green one red."—Macbeth.

Abstract

IN the tough and stirring days in which he lived, Shakespeare can scarcely have failed to notice that a dilute solution of blood is yellow in hue instead of red, but at least he had no opportunity to learn that the same change of hue is also apparent when blood corpuscles are examined under a microscope. This observation, which has been the subject of recent correspondence in NATURE ^1,2,3, appears at first sight to be one of those odd effects which can only be explained by reference to some peculiarity of vision. Actually it is an interesting and important example of the general phenomenon of dichroism, an all-embracing term used to describe the changes in hue which some media and surfaces undergo in certain conditions. Thus dichroism includes changes in hue which are observed with some media, such as blood, when their thickness or concentration is varied, the very striking and dramatic changes in colour of certain dyed materials when viewed successively in daylight and in artificial light, and the undertone of a pigment which is observed to differ in hue from that of the mass of the pigment when the latter is greatly extended with white. While it is true, as will be seen later, that certain visual factors are involved, yet the phenomenon has an essentially physical origin which is revealed when calculations are made to determine the spectral composition of the light reaching the eye.