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



V 15'' 30"\ a quantity far too great to be considered an error 

 of observation. 



The same spot disappeared 12'' 2''; and having come into 

 view Feb. 27<' 13" SO-", leaves 12'' 8" 30'" for the time they were 

 visible. 



Whether this be the true period, or the spots have a proper 

 motion of their own, subject to no known regular laws, which 

 sometimes lengthens the time they are invisible (but then it 

 ought as often to increase the time during which they are in 

 view) must be determined by observing them when upon the 

 very edge of the disc ; and this will rarely be observed at the 

 entrance and exit of the same spot. Should the period which I 

 have deduced be proved to be the true one, Sir VV. Herschell's 

 hypothesis cannot be supported ; for the luminous stratum forms 

 what is called the sun's disc : below this are the opaque clouds, 

 and below these the dark globe ; therefore, the nucleus ought 

 first to disappear, which is not the case. It certainly may be 

 seen when the umbra becomes invisible by reason of its proximity 

 to the edge of the solar disc : indeed in 1818 I traced a spot 

 which was surrounded with a fine umbra to the very edge ; when 

 there was a fine line of light beyond the spot, both the nucleus 

 and umbra were very distinct ; about half the nucleus projected 

 beyond the umbra towards the sun's center. According to his 

 hypothesis, the nucleus should disappear first, and even before 

 it comes very near the limb, which is contrary to observation ; 

 then the umbra should disappear upon the limb, and return after 

 13i-days. His hypothesis does not explain the reason why the 

 margin of the umbra is darker than the interior, nor the clouded 

 mottled form which large umbrae commonly assume. 



I do not presume to produce any hypothesis ; their structure 

 is very remarkable, and totally dissimilar to that of any objects 

 with which we are acquainted. When viewed with a telescope 

 of sufficient power and great distinctness, their form presents 

 many inexplicable phenomena ; they are best seen with a large 

 aperture, and power of from 70 to 200. Two pieces of plate 

 glass, each smoked on one side, glazed with the smoked sides 

 towards each other, interposing a margin ol' card paper to pre- 

 vent their touching, is a preferable screen to any of the coloured 

 glasses ; and if made sufficiently large, the glass is not liable to 

 be split by the heat, an accident to which 1 have always found 

 coloured glass fixed into a brass ring so liable, that I can rarely 

 use such a glass more than two or three times, allowing a suffi- 

 cient aperture to the telescope. The mottled appearance of the 

 solar disc, first particularly noticed by Sir W. Herschell, may 

 be constantly seen with a considerable aperture, if the telescope 

 be very distinct ; if a screen of very white paper be placed 

 where the focal image is formed by a good lens of about 50 feet 



