ONE ELEVEN-YEAR PERIOD OF SUN-SPOT OBSERVATIONS. 17 



telescope furnished in this way, I began to make observations on the sun in 

 March, 1867. For several days no spots were seen on the solar disk, but on the 

 7th of April I saw twenty small spots in three different clusters, or groups. In 

 this observation I used a magnifying power of 200 diameters. And here I would 

 say that a high power requires a larger aperture, or opening, in the cap. I have 

 used a cap with several slides, having apertures for different powers. The slides 

 are round and slightly fastened by one edge to the cap, so they can all be moved 

 and thus get the right size. 



With a power much above fifty the sun appears larger than the field of view 

 in the telescope, and hence cannot be all seen at once. I looked at the sun every 

 few days, generally with a power of 100, but saw no more spots till May 24th, 

 when four small spots were seen in one group near the eastern edge of the sun. 

 On the 26th several more spots were visible, two of them being larger and more 

 prominent than the rest. They were also -farther from the edge of the sun. By 

 watching them day after day, I observed that they appeared to pass across the 

 sun's disk from east to west — being a visible proof of what I had read, that the 

 sun turns on its axis. One of these spots vanished near the sun's center about the 

 1st of June; but the other remained very prominent, and seemed very near the 

 west edge of the sun on June 4th. Seeing that a spot traverses the^solar disk 

 (one side, or half of the sun) in twelve or thirteen days, we learn that he com- 

 pletes an entire revolution in about twenty-five days — the exact time being twenty- 

 five days and ten hours. 



My observations were quite frequent — sometimes every day — through the 

 summer of 1867, and but very few spots were seen, often none at all. In the 

 middle of September a group of fifteen spots appeared near the sun's center, but 

 some of them soon vanished. One, the largest I had yet seen, was at the west 

 edge of the sun on the 20th. It appeared long and narrow — as large spots always 

 do at the sun's edge 



Observations were not continued through the winter. On March 8th, 186S, 

 five spots were visible, and on the 15th twenty-five had formed in two groups. 

 Same number on the 18th, five of which were large and prominent, while several 

 were so little as to be just perceptible. But a solar spot must be 400 or 500 miles 

 across to be seen even as a speck. In four days they all had vanished — disap- 

 peared before reaching the sun's edge. On the 31st there were two distinct 

 groups, one of them north of the sun's equator, the other south, both containing 

 twenty-seven spots. Near a dozen of these were quite prominent, some of them 

 encircled with a penumbra, which looks like a border surrounding the umbra 

 (black spot). One of these spots was quite large, being of a sharp triangular 

 shape at first, then oval,, oblong, etc. 



Throughout the months of April, May and June, 1868, from twenty-seven to 

 thirty-four spots were visible nearly every day. For several days in the fore part 

 of July the sun was clear of spots. Eight appeared on the 2 2d, and 30 — all in one 

 group — on the 25th. But few were visible in August or September. On the 



2— in. 



