142 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



luted oxygen the law is quite different. The elastic force 

 of the vapor of phosphorus is 0.5 millimeters at a temper- 

 ature of 5 Centigrade, but is 6.8 millimeters at a tempera- 

 ture of 40 Centigrade. In absolute vacuum, or in pure inert 

 gases, no phosphorescence takes place. 8 B,July 25, 1874, 

 87. 



OX OPTICAL niENOHENA AT THE TRANSIT OF VENUS. 



Mouchez states in reference to the observations of the 

 black drop, as it appeared to the French observers at the 

 island of Saint Paul in the Southern Pacific Ocean, that in 

 proportion as Venus entered upon the sun's disk the clouds 

 became thinner, the heavens more transparent, and images 

 of great precision. A quarter of an hour after the first con- 

 tact, while half of the planet was still beyond the limit of 

 the sun, suddenly the whole disk of Venus was perceived 

 shining like a pale circle, more brilliant in the neighborhood 

 of the centre than at the outer edges of the planet. The 

 diameter of the planet was immediately measured with the 

 micrometer, and found to be identical in all directions through 

 its centre, showing that the disk was real, and not an optical 

 deception. But in proportion as the second contact ap- 

 proached the two brighter portions in the neighborhood of 

 the centre tended to unite themselves into an enveloping 

 circle of bright light around the segment of the planet which 

 was still exterior to the sun. And this anticipated reunion 

 of the horns by a luminous circular arc was rendered more 

 complete still by a small, very brilliant flange of light termi- 

 nating the aureole on the disk of Venus. Foreseeing that 

 there would be very great difficulty, if not impossibility, in 

 observing the geometrical second contact, the observer 

 changed quickly the pale blue colored glass for a darker one, 

 by the aid of which he hoped to avoid this aureole, and these 

 accidental glimmerings. But this was useless ; the aureole 

 remained always visible, and he was obliged to return to 

 the original lighter-colored glass. Under these conditions, 

 he took as the moment of contact, not the reunion of the 

 two horns or the geometrical contact, but rather the moment 

 when the disk of the sun seemed no longer to be deformed 

 by the brilliant light which enveloped the planet at the 

 point of contact. Pie noted a very sensible difference of 



