For this purpose the eyepiece is to be dra\%n out somewhat, and 

 whether the Davis Screen or card is used a position can be found 

 by experiment where the image can be sharply focused. 



It would seem that the card system would offer some incon- 

 venience in trying to follow the sun with two tangent screws and 

 hold the card at the same time but it has been customary to clamp 

 the instrument when the telescope has been approximately set, then 

 by setting either wire a little ahead of the sun's apparent motion, 

 keep the other wire tangent to his image and allow the motion of 

 the sun to arrive at the other position of tan gene y. 



Theory 



Referring to Fig. 90*, O is the position of the observer, Z is the 

 zenith directly above him, P is the celestial pole at an altitude above 



ZENITH 



SC'JTH 



P-POLE 



NORW 



Fir. 90 



the north horizon equal to the latitude of the place of observation, 

 and S, the sun as it would appear in the morning in north declina- 

 tion. The line SO should properly be drawn to the center of the 



* Louis Ross, I '. .V. l)<f>uty A 



in Kn K . AV M 1 original observations on th is suh- 



iect< st Harvard nf. Jour.. Jan. 1910. 



153 



