Observations of the Trcmsit of Mercuri/. 197 



Description of Phases. 



a. Indentation suspected. 



b. Indentation sure. 



c. Indentation between phases (1) and {'2)D. O. Diagram. 



d. Instrument very unsteady. 



e. Light closes around planet. No " black drop." 



f. Uncertain. Large " black drop.'' 



g. Sudden breaking of thread of light. No black 

 drop." 



h. Last indentation surely seen. Sun's limb serrated. 



i. Last indention suspected. 



j. Indentation size of phase (2) on D. 0. Diagram. 

 k. Indentation last surely seen. 



The two observers, though separated by only about 

 twelve feet were entirely independent and unbiased; they 

 being mutually invisible without change of position. 



The instrumental times recorded are those given by the 

 Union College Dent mean-solar chronometer beating half 

 seconds, which was placed midway between the two ob- 

 servers, to both of,whom the sound of the beat was plainly 

 audible, and for whom the count was continually verified 

 by Mr. Johnson by ticking off every tenth second. 



Professor Olds assisted Mr. Wilson by identifying the 

 decades of seconds and the minutes, and recording his 

 observations ; while I kept my own record and count, 

 verifying the latter both before and after each contact. 



The unsteadiness of the Dolland glass at ingress caused 

 by wind, in its unstable and exposed position, together 

 with the difficulty of changing an alt-azimuth motion into 

 a uniform equatorial one, rendered the observation of the 

 first contact somewhat unsatisfactory to Mr. Wilson, but 

 with the Clark instrument, free from these disturbing 

 causes, the planet appeared in contact at the center of the 



