OF ARTS AND SCIENCES. 



245 



The total brilliancy was observed several days later, — the day 

 before full moon. Time, 9 p.m., altitude, 20°. Observations were 

 made with both the Bunsen and the Rumford photometers, and are 

 given in full below. Unit, .1 of an inch. 



Bunsen. 



C. Side. 



883 



850 

 882 

 956 

 983 



911 



1,002 

 790 



896 



212 



Rumford. 



Candle-power at 1 Meter's Distance. 

 Bunsen, .187. Rumford, .124. 



The observations with the Bunsen were made from both sides of 

 the disc. In those marked C side, I placed my eye on the side of the 

 candle, in the other it was on the side of the moon. The two means 

 agree very closely; but it was noticed that when the yellow light of 

 the candle passed through the oiled paper, the spot almost completely 

 disappeared; on the other hand, when it was reflected directly from 

 the surface, the setting was much more difficult to make. This 

 difference was very marked, and an examination of the results will 

 show that those made on the side of the moon agree much better 

 than those made on the other side. I shall refer to this point again 

 when T come to the measurements of the sun. On using the Rum- 

 ford photometer, I was struck with the fact that the measurements 

 did not at all agree with those made by the Bunsen. They agreed 

 with each other, however, more nearly than those made by that 

 instrument, and the difference between their limits was less. 



I then set the screen at the mean of the Bunsen readings, but 

 could not convince myself that the shadows were equally dark. The 



