by a halo which nearly obliterates the carbons.  This arises from an imperfection of the glass lens, called its spherical aberration, which is due to the fact that the circumferential and central rays have not the same focus.  The human eye labours under a similar defect, and from this, and other causes, it arises that when the naked light from fifty cells is looked at the blur of light upon the retina is sufficient to destroy the definition of the retinal image of the carbons.  A long list of indictments might indeed be brought against the eye—­its opacity, its want of symmetry, its lack of achromatism, its partial blindness.  All these taken together caused Helmholt to say that, if any optician sent him an instrument so defective, he would be justified in sending it back with the severest censure.  But the eye is not to be judged from the standpoint of theory.  It is not perfect, but is on its way to perfection.  As a practical instrument, and taking the adjustments by which its defects are neutralized into account, it must ever remain a marvel to the reflecting mind.

Sec. 3. Rectilineal Propagation of Light.  Elementary Experiments.  Law of Reflection.

The ancients were aware of the rectilineal propagation of light.  They knew that an opaque body, placed between the eye and a point of light, intercepted the light of the point.  Possibly the terms ‘ray’ and ‘beam’ may have been suggested by those straight spokes of light which, in certain states of the atmosphere, dart from the sun at his rising and his setting.  The rectilineal propagation of light may be illustrated by permitting the solar light to enter, through a small aperture in a window-shutter, a dark room in which a little smoke has been diffused.  In pure air you cannot see the beam, but in smoky air you can, because the light, which passes unseen through the air, is scattered and revealed by the smoke particles, among which the beam pursues a straight course.

The following instructive experiment depends on the rectilineal propagation of light.  Make a small hole in a closed window-shutter, before which stands a house or a tree, and place within the darkened room a white screen at some distance from the orifice.  Every straight ray proceeding from the house, or tree, stamps its colour upon the screen, and the sum of all the rays will, therefore, be an image of the object.  But, as the rays cross each other at the orifice, the image is inverted.  At present we may illustrate and expand the subject thus:  In front of our camera is a large opening (L, fig. 2), from which the lens has been removed, and which is closed at present by a sheet of tin-foil.  Pricking by means of a common sewing-needle a small aperture in the tin-foil, an inverted image of the carbon-points starts forth upon the screen.  A dozen apertures will give a dozen images, a hundred a hundred, a thousand a thousand.  But, as the apertures come closer to each other, that