174 LOWELL — SUPPOSED SIGNALS FROM MARS. [Dec. 6, 



The results are : 



Projection December 7, 1900. 



Table I. 



Date. 



Dec. 7 



and for 



Projection December 8, 1900. 



Table II. 

 Date. h. m. P. A. d. 



Dec. 8 15 50 186O.3 ^thread 



16 25 188O.1 \y 2 « 



16 44 187O.1 y 2 « 



15. The numerical value of d was got as follows: Mr. Douglass' 

 micrometric measures of the spider's thread were obtained by 

 moving the movable thread from contact on the one side of the 

 stationary thread to contact on the other. This gave o".2 2 for 

 the width of a thread. 



In the estimating of the distance d the thread was placed against 

 the background of the disk. As a measure of width it was there- 

 fore its true width less the irradiation into it from both sides. The 

 value of this irradiation was determined by the following device 

 which occurred to me, and which if accurately made should give 

 the irradiation absolutely. From the point of contact the thread is 

 moved till the bright background seems equal to the width of the 

 thread. We then have the following equation, in which 



a = width of thread. 



b = width moved from contact to one apparent thread apart. 

 a~ = the irradiation. 

 a — 2.x = b -\- 2.x. 



or 



a — /' 



4 

 b proved =0". 14 



whence the effective width of the thread was o".i8. 



The P.A.'s used were got either directly or by plotting all the 

 P.A.'s taken and then drawing the centre of gravity line between 

 them. 



