Hi. M. Parkhurst on the Photo-mapper. — 
guishable from the visible portion of the disk of a star, I have 
found it necessary to use a blue shade. I have extended the 
telescope by various tubes, sometimes nearly two feet; but with 
‘the use of the slides I find that a tube five inches long to hold 
the eye-piece, usually gives sufficient extension to the telescope 
and sufficient range of brightness. The amount of the exten- 
sion I measure with dividers, using a prepared scale which 
to determine the aggregate brightness of the two stars. It may 
be used for measuring the brightness of planets, small nebule 
different process. ; 
The error of the determination of the magnitude of a star 
by the method of equalizing disks, may be divided into four 
arts : 
The error of the assumed magnitude of comparison stars. 
In my series of observations, instituted to determine the rela- 
tive amount of the several errors, this error hardly exceeds ™-01. 
Il. The error of observation, or inaccuracy of judgment as to 
the exact point where the disks are equalized. Not only accor- 
ding to the statements of Arago, Silliman, and Crookes, redu- 
ced by myself, but aecording to my own results, the error of 
