142 ANNUAL OF SCIENTIFIC DISCOVERY. 



even some which, in a very few hours, pass from the second to the 

 fourth magnitude ; and there are others in which the changes in inten- 

 sity are much more decided. These stars, quite visible at certain 

 epochs, totally disappear, to reappear in periods longer, or shorter, 

 and subject to slight irregularities. Two explanations of these curious 

 phenomena present themselves to the mind ; the one consists in sup- 

 posing that the star is not equally luminous on all parts of its surface, 

 and that it experiences a rotatory movement upon itself; thus it is 

 brilliant when the luminous part is turned towards us, and dark when 

 the obscure portion arrives at the same point. According to the other 

 hypothesis, an opaque, and, in itself non-luminous satellite, circulates 

 round the star, and eclipses it periodically. In accordance with one 

 or the other of these suppositions, the light which is exhibited some 

 time before the disappearance, or before the reappearance of the star 

 has not issued from all points of the circumference. Hence, there can 

 be no doubt of the complete neutralization of the tints of which we 

 have just spoken. 



If a changing star, when examined by a polarizing telescope remains 

 perfectly white in all its phases, we may rest assured that its light 

 emanates from a substance similar to our clouds, or our inflamed gas. 

 Now, such is the result of the few observations that have been hitherto 

 made, and which will be highly useful to complete. This means of 

 investigation demands more care, but succeeds equally well, when 

 applied to those stars which experience only a partial" variation in 

 their brilliancy. The conclusion to which these observations conduct 

 us, and which we may, I think, without scruple generalize, may be 

 announced in these terms ; our sun is a star, and its physical constitu- 

 tion is identical with that of the millions of stars with which the firma- 

 ment is strewed. 



DESCRIPTION OF A NEW EYE-PIECE FOR THE SUN, WITH 



REMARKS ON SOLAR SPOTS. 



THE following is a report of some remarks recently made at the 

 London Astronomical Society, accompanied with the exhibition of a 

 new solar eye-piece, by the Rev. W. 11. Dawes. The principal pecu- 

 liarity of this eye-piece consists in a metallic slide, with perforations of 

 different sizes, which crosses the eye-tube at right angles, just at the 

 focus of the object-glass. There are contrivances, which can easily be 

 imagined, for rapid manipulation, and though the slide is of course 

 greatly heated when the sun is viewed, the conduction is cut off by 

 interposing a plate of ivory. The perforations vary in diameter from 

 0.5 to 0.0075 of an inch ; and, with a small field, single lenses are pre- 

 ferred to complicated eye-pieces. Mr. Dawes finds that, in general, 

 apertures which exceed 0.3 inch cannot be safely used for a long time 

 under a hot sun. Where the usual proportions are retained between 

 the aperture and focal length of an object-glass (seldom exceeding 1 

 to 1G in large telescopes,) a focal diaphragm of (.;> inch transmits a 

 portion of the sun's image, the size and heat of which is nearly equiv- 



