It appears to be a large glowing disc, sensibly circular in outline, with its edge fairly well-defined both as seen in the telescope and as registered on photographs. In the spectroscope, or when in an eclipse of the Sun the Moon covers the whole disc, a narrow serrated ring is seen surrounding the rim, like a velvet pile of a bright rose colour. This crimson rim, the sierra or _chromosphere_ as it is usually called, is always to be found edging the entire Sun, and therefore must carpet the surface everywhere. But under ordinary conditions, we do not see the chromosphere itself, but look down through it on the _photosphere_, or general radiating surface. This, to the eye, certainly looks like a definite shell, but some theorists have been so impressed with the difficulty of conceiving that a gaseous body like the Sun could, under the conditions of such stupendous temperatures as there exist, have any defined limit at all, that they deny that what we see on the Sun is a real boundary, and argue that it only appears so to us through the effects of the anomalous refraction or dispersion of light. Such theories introduce difficulties greater and more numerous than those that they clear away, and they are not generally accepted by practical observers of the Sun. They seem incompatible with the apparent structure of the photosphere, which is everywhere made up of a complicated mottling: minute grains somewhat resembling those of rice in shape, of intense brightness, and irregularly scattered. This mottling is sometimes coarsely, sometimes finely textured; in some regions it is sharp and well defined, in others misty or blurred, and in both cases they are often arranged in large elaborate patterns, the figures of the pattern sometimes extending for a hundred thousand miles or more in any direction. The rice-like grains or granules of which these figures are built up, and the darker pores between them, are, on the other hand, comparatively small, and do not, on the average, exceed two to four hundred miles in diameter.