

TAKING OBSERVATIONS. 



precisely how far his visual faculties vary from the ideal. Hence arises a question, incom- 

 prehensible to the uninitiated, which, however, is commonly asked among astronomers themselves 

 f What is the value of your personal equation ? ' This inquiry is answered by a figure 

 expressing the particular amount of deviation from the standard. The most singular thing is, 



THE GREAT EQUATORIAL TELESCOPE IN THE DOME, GREENWICH OBSERVATORY. 



that the value of the personal equation is different in the same individual as regards the 

 various celestial bodies. Some can very quickly discern the phenomena of a fixed star who 

 are much slower in perceiving those of the moon, and vice ver&d. In order to obviate the 

 inconvenience which might result from the variations in personal equations, they also have 

 recourse to a very ingenious plan. An eye-piece with two tubes allows two assistants simul- 

 taneously to observe the passage of the same star over the same threads in the instrument ; 

 156 



