382 Rev. Mr. Emmett on the Solar Spots. [May, 



in consequence of the imperfection of the telescope, before the 

 reflecting and achromatic ones were invented : however, we must 

 bear in mind that Cassini, and some others quoted, were good 

 observers ; their telescopes showed the division of Saturn's ring 

 — his belts — the spots on Jupiter, and shadow of his sateUites — 

 the spots of Venus and of Mars, as well as many other of the 

 most dehcate objects, some of which, as the spots of Venus, are 

 not easily seen, even with our improved instruments ; besides, 

 the telescopes then in use could trace a spot to the very edge of 

 the sun's disc ; than which more cannot be desired, especially 

 since they showed the entire structure of the spots, which 

 requires distinctness of vision. Also I have lately constructed 

 two aerial telescopes, precisely such as were formerly used ; one 

 has an aperture 1-7 inch, and focus 18 feet; the other an aper- 

 ture 3 inches nearly, and focus near 50 feet : the former has 

 powers of 35 and 70; the latter 95 and 190; in distinctness 

 they are very nearly, if not quite equal, to an excellent Newto- 

 nian reflector of six inches aperture, which shows the quintuple 

 belt and double ring of Saturn, with a power of 800, and 

 showed the spots of Venus, Feb. 21, of the present year, with 

 powers 120,200,400. The shape of the nuclei and structure of 

 the umbraj of the solar spots are beautifully seen with all the 

 instruments : indeed from observation I am certain, that the 

 long focal lengths of the telescopes whose object-glass is a single 

 convex lens, gives them an advantage of no small magnitude 

 over every other instrument in certain cases. As I shall trouble 

 you with some observations made with these instruments, when 

 those I shall have had opportunity to make are sufficiently nu- 

 merous, I shall dismiss the subject for the present, assuring those 

 who will make trial of a good plano-convex lens of crown glass, 

 of from 20 to 50 feet focal length, that they will find, under judi- 

 cious management, a distinctness of vision possessed by few 

 instruments. 



These remarks I have been induced to make, to prove that I 

 am convinced from actual and lono- continued observations, that 

 this extraordinary phenomenon is not to be ascribed to imper- 

 fection of the instruments; nor can it be attributed to any want 

 of care, for perhaps a more indefatigable and careful observer 

 than Cassini never lived ; and besides it is not a little remarka- 

 ble, that until very recently the two periods were not considered 

 to be equal, but were believed to be 12 and 15 days; yet no 

 journal, nor observations that I have seen, have been advanced 

 in support of the recently received opinion. Accordingly I have 

 instituted a series of observations; but not having yet made a 

 sufficient number to enable me to discover to what extent the 

 period may appear to vary in consequence of the proper motion 

 of the spots, I do not propose them as infallibly decisive ; I 

 wish to make them public rather for the sake of inducing other 



