In the human eye an attempt is made to apply all these methods, but the most important is the third, that of applying the diaphragm formed by the iris, a circular semi-muscular curtain lying just in front of the anterior surface of the lens. The iris is also furnished with a layer of pigmental cells which effectually stop out all peripheral rays of light that otherwise would pass into the eye, creating circles of diffusion of a disturbing nature to perfect vision. This delicate membrane, then, is kept in constant action by a two-fold nerve supply, derived from five or six sources, which it is unnecessary to describe at length. But the eye, with all its marvellous adaptations, has an obvious defect, that of secondary or uncorrected chromatic aberration. =Chromatic Aberration of the Eye.=--White light, as previously explained, is composed of different wave lengths; and accordingly as these undulations are either longer or shorter, so do they produce on the eye the impression of different colours. We have seen how a pencil of white light may, by means of a prism, be decomposed into a multi-coloured band. In an ordinary magnifying reading-glass these coloured fringes are always seen around the margins. In practical optics chromatic aberration is partially corrected by employing two different kinds of glass in the construction of certain combined lenses. In the human eye chromatism cannot be corrected in this way; hence a blue light and a red light placed at the same distance from the eye appears to be unequally distant: the red light requiring greater accommodation in the eye than the blue, and this accordingly appears to be the nearer of the two. This visual error may be experimentally shown and explained. There is a kind of glass which at first sight appears dark blue or violet, but which really contains a great deal of red. Take an ordinary microscope lamp, having a metal or opaque chimney, and drill a circular hole in it, about 3 mm. in diameter. This opening should be just at the height of the flame; cover it over with a piece of ground glass and a piece of the red-blue glass. Thus will be formed a luminous point whose light is composed of red and blue, _i.e._, of colours far apart from each other in the spectrum. [Illustration: Fig. 24.--Chromatic Aberration of Eye, showing the wave differences of the _blue and red rays of light_ (_Landolt_).] If rays coming from this point enter the eye, the blue rays (Fig. 24), being more strongly reflected than the red, will come to a focus sooner than the latter. The red rays, on the contrary, will be brought to a focus later than the blue, while the latter, past their focus, are diverging. Let A B C D (Fig. 24) be the section of a pencil of rays given off from a red-blue point sufficiently distant so that these rays may be regarded as parallel. The focus of the blue is at _b_, that of the red at _r_.