148 Scientific Intelligence. [Aug. 



whose period or wave is (\) involving certain constant quantities 

 dependent on the nature of the prismatic medium, H, r, n, and tt 

 the semi-circumference) is this : 



By substituting Fraunhofer's values of X from the interference 

 spectra, the values of jj, for the same rays were calculated, and com- 

 pared with the refractive indices determined also with the utmost 

 accuracy by Fraunhofer for the same rays, in prisms of ten different 

 substances ; viz. . four kinds of flint glass, three of crown glass, water, 

 solution of potash, and oil of turpentine. These calculations gave 

 an accordance in every instance exact to three places of decimals. 

 And this will be allowed to afford a complete verification of the 

 theory for all the cases examined. These in fact constitute all the 

 instances in which exact determinations have yet been made. The 

 author is therefore now engaged in endeavouring to extend the 

 series to a larger range of media. For this purpose it will be neces- 

 sary to form prisms of a variety of solid and liquid media, and con- 

 siderable difficulty may arise from not being able to get them 

 sufficiently pure and homogeneous to shew the lines. 



Again, the calculations present considerable difficulty ; especially 

 in finding a series of arcs, which shall fulfil the twofold condition of 

 being themselves in the ratio of the values of X, while they are to 

 their sines as those of ^. This problem has been facilitated by a 

 remarkable elegant geometrical construction, communicated to the 

 author by the Rev. A. Neate, M.A., of Trinity College. 



Even without reference to the tlieory of undulations, any formula, 

 were it but empirical, would in the present stage of the inquiry be 

 valuable, if it afforded an accurate representation of the facts. In 

 this point of view, then, the formula here employed may be regarded 

 as not unimportant, if it merely express the numerical law of dis- 

 persion for the substances examined. 



Ma;/ 29th, 1835.— A paper was read by the Rev. R. W. Browne, 

 M. A., St. John's College, on Halley's comet. 



The following are the recorded appearances of this comet. We 

 repeat this because some of the figures in our report of Dr. Lardner's 

 lecture are inaccurately printed. (See Vol. I. 467.) 



1. B. C. 130, birth of Mithridates. 



2. A. D. 323, after six revolutions. 



3. „ 379, see Lubienietski in Theatr. Com. 



4. „ 550, Rome taken by Totila the Goth. 



5. „ 1005, no remarkable event. After five revolutions. 



6. „ 1230, idem. After three revolutions. 



7. „ 1305, the plague. 



8. , 1456, Pope Calixtus II. Mahomet II. took Constantinople. 

 According to theory as well as history the magnitude and brilliance 

 of the comet were then at their maximum. 



9. „ 1531, observed by Apian of Ingoldstadt. 



