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the ideas are principally formed from the motion of our eyes; otherwise the different colours and brightness of objects and also the magnitudes of such as we had not seen before would lead us into perpetual mistakes.
With respect to the relative brightnesses of the same object placed at different distances, it is to be observed that they would be exactly the same, if no light were stopped or dispersed in its passage through the air; for supposing the aperture of the pupil to remain the same, the quantity of light entering through it varies inversely as the distance of the object from the eye, but the area of the picture on the retina over which that light is spread varies in the same ratio,[1] so that this picture is, in all cases, equally enlightened.
CHAP. XV.
OPTICAL INSTRUMENTS.
The common Looking-glass.
149. This instrument is not quite so simple as it seems at first sight. It consists of a plate of glass more or less thick, with a sheet of metal at the back of it. This metal, which is an amalgam of block-tin and mercury, fits tightly to the glass, and its surface being smooth and bright, reflects any light thrown on it. However the rays have to pass through the glass on their passage to and from the speculum, and therefore suffer two refractions, which do not indeed affect their general directions, if the surfaces of the glass be accurately parallel, but which have a tendency to cause irregularity in them.
This will easily be seen from Fig. 154, which is purposely drawn on an exaggerated scale. represents a portion of the
- ↑ The object and the image subtend the same angle at the center of the eye, and therefore the area of the image is to the visible area of the object, as the square of the distance from the center of the eye to the retina, to the square of the distance of the object from the eye, in which proportion the means are invariable.
