1895.] NEW YORK ACADEMY OF SCIENCES. 167 
ing the principal sections of the double prism unto coincidence 
with the horizontal and vertical directions. 
This adjustment was effected in the following way. The axis 
of the tube bearing the prism and Nicol was first set, by meas- 
urement, parallel to the cross-piece D M of the supporting frame. 
The base G was then carefully levelled by means of a common 
level. The levelling of D M then brought the axis of the tube 
into coincidence with the horizontal line. The telescope was 
then focussed upon a very fine line of light reflected from the 
edge of a carefully levelled sheet of white paper placed just in 
front of the rectangular opening O. Since the line joining the 
two images produced by a crystal of cale-spar is always paralled 
to the optical axis of the crystal, it follows, that, when the two 
images of the horizontal line of light form with each other an 
unbroken line, the principal sections of the crystal have the de- 
sired directions, and the adjustment is per fect. The line of 
light used in this case was so narrow that the adjustment was 
able to be made with great accuracy. This done, the crystal 
was permanently fastened in position. Thereafter, it was ‘only 
necessary, before each observation, to level the base of the in- 
strument G, in order to bring the principal sections of the crys- 
tal into the desired positions. 
In order to set the axis of the tube at any desired angle with 
the vertical, the cross-piece D M, parallel to this axis, was lev- 
elled,and the angle D E G, between the movable arm and the ver- 
tical support, was measured by means of a protractor. The zero 
position being thus determined, any desired inclination could be 
secured by giving to the angle D E G the proper value. 
VeEE 
DEGREE OF ACCURACY OF THE INSTRUMENT. 
The great sensitiveness of the eye in detecting slight differ- 
ences in the intensities of images of the same color, when brought 
into close proximity, has often been the subject of remark. Cornu 
claims that the position of equality can be determined with a 
precision that reaches 75 of a degree. My own observations 
would not lead me to attribute to the instrument so high a 
degree of accuracy. Furthermore these observations are subject 
to the objection which attaches to all phote-metric experiments, 
that the sensitiveness of the eye varies greatly with the physical 
and mental condition of the observer. At times the extreme 
difference in my readings for a given set of conditions would be 
as high as 24 degrees. Usually, however, the extreme difference 
was not more than 14 degrees. For the sake of testing the 
