6lO TRANS. ST. LOUIS ACAD. SCIENCE. 



With re^^ard to these observed phases, it is to be remarked that 

 phase A, as aheady indicated, was the first moment when I 

 could detect motion at the sun's edge. Using the micrometer 

 wire to mark the place of entrance, I was watching the sun's limb 

 at the exact point where the planet entered. The image was 

 steady, and I noticed no distortion or unusual phenomena as the 

 planet entered upon the sun's disc. I am led to think, for rea- 

 sons given later, that this phase for external contact corresponds 

 to phase C for internal contact. 



Phase B, when the planet's disc was just bisected by the sun's 

 limb, is capable of very exact observation. It can be noted quite 

 as accurately, and I am inclined to think more accurately, than 

 the contacts of the limbs, and its observation is free of many of 

 the personal elements of uncertainty that enter into observations 

 of the contacts. I am of the opinion that more accurate and uni- 

 form results can be obtained from the observation of this phase 

 than from the contact observations. 



Phase C was noted at the moment when the limb of the planet 

 seemed to have come into geometrical tangency with the sun's 

 limb. The two discs did not separate, however, until some seconds 

 later, although I noticed no distortion. 



Phase D marks the first moment when the thread of sunlight 

 •seemed to flash around the limb of the planet, and corresponds, 

 no doubt, to the moment of true internal contact. 



During the interval between the external and internal contacts, 

 and afterward when the planet was fully projected on the sun's 

 disc, it was studied carefully to see if any trace of an atmospheric 

 ring, or any variations in the color of the disc, could be seen. 

 Nothing of this nature could be detected either by myself or by 

 several others who looked at the planet. The planet presented 

 the appearance of a uniformly black disc projected upon the disc 

 of the sun. 



The effects of irradiation in observations of this character, and 

 the discrepancies between the observations of different observers 

 caused thereby, have given rise to much discussion in connection 

 with recent observations of the transits of Mercury and Venus. 

 During the transit which has just occurred my observations were 

 directed specially toward a study of the amount and character of 

 the irradiation effects upon the contact observations. 



