198 Astronomical and Nautical Collections. 



and this date is less likely to be affected by the error of the 

 equation than the former ; but they both require correc- 

 tion on account of the erroneous length of the year em- 

 ployed in carrying back the epoch : for though Ptolemy's 

 sidereal year is within 12 seconds of the truth, his tropical 

 year, as well as that of Hipparchus, is about five minutes too 

 long : since these great astronomers agree in making the 

 Julian year too long by -^%-^ of a day, while the Gregorian is 

 shorter by ^§-^ ; so that in the 600 years preceding the most 

 accurate observations of Hipparchus, they made this differ- 

 ence 2 days instead of 4^, and they supposed the sun to have 

 been 2^ days too far advanced on the day in question, and to 

 have described a space so much shorter than the truth. This 

 correction would give us the date ^ —746^ — 31. 55^ But the 

 mean of Hipparchus's actual observations, reduced, with all 

 possible care, according to the correct value of the tropical 

 year, gives us nearly ^ — 746^ — 30.4^ for the epoch of Na- 

 bonassar. 



With the assistance of Mr. Raima's table of astronomical 

 chronology, and of the Memoirs of Professor Ideler, which he 

 has republished, I have endeavoured to exhibit, in chrono- 

 logical order, the various observations which are scattered 

 through the works of Ptolemy, and to connect them with the 

 series of Olympiads, and with other chronological epochs. 

 But I must defer this table to a future occasion. 



Believe me, dear Sir, 



Yours very sincerely, 



Park Square, 8 March, 1828. T. Y. 



ii. Elementary View o/^^e Undulatory Theory of Light, 

 By Mr. Fresnel. 



[Continued fromthe last Number.] 

 Of coloured Rings, 

 The cololired rings, exhibited by two glasses pressed toge- 

 ther, when one of the surfaces in contact is slightly convex, 

 are explained in a very simple manner by the principle of 

 interferences : they evidently result from the mutual influ- 

 ence of the two systems of undulations reflected at the first 



