When we look at the sun through clouds, or through a darkened glass, it appears as a uniform disk of light. But, when we look at it through a telescope of moderate magnifying power, the uniformity of

surface disappears, and we see it irregularly sprinkled with dark points known as solar spots. These are, however, not always seen; sometimes the solar disk is perfectly clear. It has been found that for a period of ten years, from 1840 to 1850, out' of a total number of 1,982