PKITCHETT — ON THE TRANSIT OF MERCURY. 6l I 



For this reason I noted with as great care as possible all the 

 phases of the contact observations, and also the moment when 

 the sun's limb appeared to bisect the disc of the planet. This 

 last phase should be free of the effect of irradiation, and a com- 

 parison of it with the contact observations should give some idea 

 of the effect of the irradiation upon the contact observations. 



The effect of irradiation would be to diminish the apparent 

 disc of the planet as seen in projection on the sun's limb. In con- 

 sequence the observed external contact of this disc with the sun's 

 limb would come later and the internal contact earlier than those 

 of the real image of the planet as unaffected by irradiation. 



Referring now to the observations of phases A and C, there 

 can be no question, I think, that they refer to the external and 

 internal contacts of the planet's disc as affected by irradiation. 

 The mean of the two times is ii//. S5''^- 2zs., which agrees close- 

 ly with the observed time of that phase. 



Between observations C and D there was no apparent distoi-- 

 tion of the planet's disc, but the planet seemed to cling to the 

 edge of the sun. The observation noted D is the instant when 

 the light first ffashed between the limb of the sun and the planet. 

 Assuming this to be the moment of true internal contact, the 19.4 

 seconds between C and D would measure the effect of irradiation 

 upon the contact time. This would correspond to a diminution 

 of the planet's semi-diameter of o".Si. 



After the planet had entered fully upon the sun's disc, its 

 diameter was measured with the filar micrometer, with the fol- 

 lowing results : — 



For position angle 0° diameter ^ 10". oS 

 For position angle 90° ■' "=9 -83 



These have been corrected for the differential refraction. The 

 mean of these results would give for the apparent semi-diameter 

 4". 98. The angular semi-diameter in use both in the Berlin 

 Jahrbuch and the American Ephemeris is that of Bessel, deter- 

 mined with the heliometer, and therefore free of irradiation. In 

 A. N., No. 3034, Mr. L. Ambronn gives the result of all the best 

 known determinations of the diameter of Mercury, both those 

 obtained from double-micrometer methods and those resulting 

 from measurements with filar micrometers. The result from all 



