280 Transactions of the American Institute. 



able decrease lias been detected. Few observations available for the 

 purpose have been bequeathed us by the astronomers of former days ; 

 but from some careful observations made in 1801 by the distinguished 

 astronomer Olbers, upon the brilliancy of Mars, which depends, of 

 course, upon reflected solar light. Prof. Seidel, of Munich, was able 

 to prove in 1859, that the solar light had not diminished in tlie inter- 

 val to any measurable extent. It has been well said, that had some 

 of the old Greek metaphj^sicians once dipped a thermometer in the 

 Kissus, and recorded the date and temperature, it M'oukl have added 

 a thousand fold more to our knowledge than all their speculations 

 did. We can forgive them for this omission, if for no (5ther reason 

 than that they had no thermometers to dip ; but had they ever 

 recorded the average date of flowering of any common plants, we 

 should be able to thank them for at least some materials whence we 

 could deduce valuable geological and even cosmological knowledge. 



The Sun's Light. 



In addition to the study of the cpiantity and intensity of light, two 

 modes of investigating its qualitj- are known to us. One is by dis- 

 covering in what planes the luminous undulations occur, the other is 

 by determining the proportion of rays each degree of refrangibility 

 •which it contains. By the first method, viz. : By the use of the 

 polariscope, we are generall}^ enabled to discover if light reaching us 

 has been reflected, or directly emitted by the luminous body, and if 

 that body is solid, liquid or gaseous. By the second, viz. : B}^ the 

 use of the spectroscope, we are enabled to analyze each ray of solar 

 light, and to infer from what incandescent chemical elements it comes, 

 at what temperatue it has been emitted, and through what forms of 

 vapor it has passed. Few sul)jects are more fascinating than these, 

 but I may not stop to set forth the processes involved, or to speak 

 of the still active progress of discovery in these directions. 



Thus far, ladies and gentlemen, I have endeavored to give you a 

 sketch of the principal facts which have been discovered concerning 

 the sun. Before asking you to give a very few moments' attention 

 to a summary of the best received theories to account for the origin 

 of his liglit and heat, find the prol)ablo length of time during which 

 they have already existed and will contimie to exist, I will say a few 

 words on eclipses, and show you ui)on the screw various representa- 

 tions of them. 



