40 HI. M. Parkhurst on the Photo-mapper. 
classes: those above the 65 being approximately measured 
by the condensed scale, and being so few in number that they 
may be conveniently, as well as more accurately, measured ,by 
a different method, to be explained below; those between the ; 
65 and the 9™5, which may be equalized in brightness with 
an artificial star in the field previously brought to an equality 
with a 95 star; and those fainter than the 9°"5, which may be § 
extinguished. The outer diaphragm will so diminish the aper- 
ture that 125 stars will be the smallest which can be seen. - 
any, more accurate than extinction by reducing the aperture. 
But comparison of disks, especially for the stars visible to the — 
naked eye, is the most accurate means of measurement known 
to me, the error of the comparison being in m experience less 
: es 8 the latter much 
of producing the final adjustment by further polarization, the 
di 7 disks render- 
