26 



whicli is the value computed by équation (12). 



Before applying équations (13) and (13'), we must give some détails of 

 the détermination of the important constant M, which is really the différence 

 in scale value corresponding to the coordinates measured along the m and n 

 Unes. The détermination of this constant was attended with great difficulty, 

 becausc a small portion of the film had been worn away at one of the points 

 to be observed. In conséquence of this circumstance, there was a great différ- 

 ence between the values of M determined by the two observers, from compar- 

 isons of line 43 with une 14. The values actually obtained for M from 

 comparisons of line 43 with line 14 were: 



ilf= + 0.95 weight 1, Woods. 



+ 0.98 weight 2, Woods. 



+ 2.87 weight 1, Goodman. 



+ 3.78 weight 2, Goodman. 



+ 2.17 weight 1, Gill *). 



The last observations were mady by Dr. Gill personally, on account of the 

 great systeraatic différence between Messrs. Woods and Goodman. The mean 

 by weight of ail the observations is: 



*j A nearly circulai- patch of tilm 3 to 4 mm. in diameter had dropped off near the 

 required point of intersection of the two lines. One observer had bisected the critical end of the 

 line above the broken tilm, the other the end below it. The terminais of both lines were somewhat 

 unsymmetrically pointed in opposite directions and there was a slight want of perfect parallelism 

 between the spider line of the microraeter and the mean reseau line. In my measures I naturally 

 took the lines on both sides of the exposed glass into account (neglecting the pointed ends) in 

 estimating the bisection. 



