40 



PROCEEDINGS OF THE AMERICAN ACADEMY 



these were not arranged with a view to eliminate the possible errors 

 involved in a and Az", as it is the intention to do in the future, the 

 results cannot be considered as having other than an illustrative value. 

 The angle, 60°, was less than should properly be used for advan- 

 tageous results, and the record gives no means of knowing in which 

 series the plate was in the direct, and in which it was in the reversed 

 position. The results are as follows. 



TABLE XIII. 



These results are of interest for comparison with those obtained on 

 the same days by Professor Rogers from different observations. The 

 series is not sufficient to determine the " irradiation constant." 



In conclusion, the results may be summarized as follows : — 



1. Observations of the four contacts by six observers. 



2. The determination, by a photometer esiDecially devised for the 

 purpose, of the relative amounts of light received from the disk of 

 Venus, from the sky near the sun's edge, and from the sun's centre. 

 Denoting the last amount by 100.0, that received from Venus was 1.6, 

 and that received from the sky 7.5. Contrary to expectation, Venus 

 was thus shown to be distinctly darker than the adjacent sky, and this 

 result was confirmed by direct observation. 



3. The spectroscojoic observations. These gave negative results, 

 and showed that no marked absorption was caused by the atmosjihere 

 of Venus. 



4. A careful determination of the diameter of Venus by a method 

 not previously attempted, and the suggestion of an application of this 

 method to planets when both limbs cannot be observed. The result 

 obtained by Professor Rogers was 16".10 from transits over inclined 

 lines, and that obtained by Mr. Chandler was 1 6".35, which would be 

 reduced 0".02 by using only the transits over inclined lines. 



