C. GENERAL PHYSICS. U5 



the air that has been mixed with the gas. 19 C\ October 25, 

 1873,404. 



THE PHENOMENA OF THE BLACK DROP. 



The numerous observations that have been made prelimi- 

 nary to the approaching transit of Venus have, among other 

 things, been quite generally directed to the explanation and 

 exact effect of the phenomenon called " the Black Drop." 

 This term is applied to a dark band sometimes appearing like 

 a pear-shaped drop, which, at the beginning and the end of 

 the transit, apparently joins together the round disk of the 

 planet and the nearest edge of the solar disk. Bakhuyzen, of 

 Leyden, describes an apparatus constructed by him (similar 

 to that established by Struve, for the purpose of comparing 

 together the observations of the Russian observers), with 

 which he was able to reproduce, at will, the black-drop phe- 

 nomena. As a result of his measures, he states that .the forma- 

 tion and breaking up of the black drop appeared simultane- 

 ously, no matter what the size and magnifying power of the 

 telescope. The smaller the diaphragm applied to the tele- 

 scope, the broader appeared the black band ; and this observa- 

 tion led him to the idea that the formation of this drop is 

 in great part the result of the diffraction of the narrow band 

 of rays of light on the edges of the object-glass, or of the dia- 

 phragms within the tube of the telescope. This idea he de- 

 velops with considerable success, and is led further, theoreti- 

 cally, to the prediction of that which he subsequently observed 

 accurately : namely, that small variations in the brightness of 

 the solar disk, or in the sensitiveness of the eyes, could exert 

 a great influence upon the apparent magnitude of the black 

 band. Bakhuyzen also deduces a formula giving the effect 

 of diffraction at other phases of the transit of Venus, and 

 shows that, between the limbs of the sun and the planet, one 

 ought to see a small dark cloud, whose brightness should be 

 the least where the limbs of the sun and Venus are nearest 

 to each other. Astron. JYcich,, LXXXIIL, 306. 



THE SPECIFIC HEAT OF GASES. 



Amagat has undertaken to determine the ratio of the 

 specific heats of gases under constant pressure, by a method 

 somewhat different from that adopted by others ; and which 



G 



