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C. & Hastings—Optical Constants of Glass. 271 
In the focal plane of the positive eye-piece was a reticle con- 
sisting of lines on glass ruled as follows :—a system of two 
airs of parallel iines crossing at center of field, one pair being 
orizontal and the other vertical, the angular distance (measured 
from objective) between the components of each pair being 
about 1’; and a single line, vertical and about 25’ to the right 
of the vertical pair. Over this single line was placed a small 
totally reflecting prism, one of its faces turned toward an open- 
ing in the eye-tube. The advantage of this arrangement is at 
once evident, for it was possible to get a strongly illuminated 
image of the single line, reflected by a plane beyond the 
objective, between the two vertical lines, at the same time 
avoiding the annoyance of waste light reflected from the eye- 
lenses on the one hand, since the mirror was within the eye- 
piece, and from the objective on the other, owing to the eccen- 
tric position of the mirror. : 
instrument was put in adjustment by means of a piece 
of plane-parallel glass placed vertically upon the platform, ie, 
so that its plane was parallel to the axis of the instrument. 
Then the glass was turned 180°. If the image of the horizontal 
lines corresponded with the lines themselves it was aes ; if not, 
the correction was divided between the glass and the telescope. 
Proceeding in this way, it was easy to adjust the telescope so 
that once sighting the reflected image it would be again visible 
i i lass 180°, and that inde- 
pendently of the azimuth of the platform as referred to the 
axes of the instrument and platform are parallel, while that of 
the telescope is perpendicular to both. The proper adj ustment 
of the collimating telescope as regards focus and direction, is so 
evidently attainable from these that it is not worth while 
describing it spevially. 
i i i ” in the prism 
same intensity, and that a motion of but 30” in 
would carry the reflected line from one of the pair to the other. 
