OF ARTS AND SCIENCES. 223 



ordinary plate, the size of a cent, fastened to its centre, could be used 

 either as a receiving or sending disc over a short line ; but it did not 

 articulate very well ; this defect, however, may have been caused by 

 the want of homogeneity in the mica. The results of my experiments 

 show that we can remove nearly one third of the iron plate (the whole 

 plate being two inches and a half in diameter) without seriously injur- 

 ing the articulation ; and, furthermore, that a piece of iron three quar- 

 ters of an inch in diameter is sufficient to cause a plate of mica two 

 inches and a half in diameter to articulate partially and sufficiently to 

 be heard over a short line. 



No. XIX. — ON A STANDARD FOR ESTIMATING THE 

 AMOUNT OF LIGHT REFLECTED BY VARIOUS 

 SUBSTANCES. 



By A. H. Lee. 



Presented Dec. 10, 1879. 



The following experiments were undertaken with the view of ob- 

 taining a standard reflecting surface, with which the light reflected 

 from various reflecting surfaces could be compared. I have collected 

 various experiments with white paper, tinted paper, and silvered 

 surfaces. 



At first I used the photometer due to Professor Pickering, and de- 

 scribed in his work on Physical Manipulations, Vol. I. p. 132. L is 

 a candle ; a and j3 are two mirrors, which reflect the light of the can- 

 dle to tho Bunsen disc A, which slides between the mirrors a and ft. 



A^ 



a --' 



•^ a 



<*V~ : — 1- - r ---^ P 



Fig. 1. 



If I represents the reflection from mirror a, and /' that from p 1 , we 

 have at A 



1 a P . or /=(£+»£ J». 



(a + b)~ (a -h c)- ' (a -+- c)- 



