164 H. M. Parkhurst on a Star-Mapper. 
tial motion with the same field, and the error from any me- 
chanical inaccuracy is therefore proportionally less. 
. Several of my early plans involved the motion of the 
star to a fixed point in the field. I abandoned this, substi- 
tuting the motion of the point to the star, for two reasons. 
(1.) It required a continual motion of the eye-piece, (or tel- 
escope,) to bring within the field the stars to be mapped ; for 
while mapping a northern star, a southern star which ought to 
mapped would be outside of the field. 
(2.) It brought within the field twice as many stars as I 
desired to map, with no line of demarcation between them. 
In 1860, thinking that I had worked with my pine mappers 
and narrow paper long enough, the instrument being readily 
adjustible and convenient for mapping in any desired position, 
I procured my first permanent star-mapper, chiefly of brass, 
- adapted to map 90’ in declination upon the scale of the Cha- 
cornac charts. In 1863, desiring to make charts upon twice 
that scale, I had another and larger mapper constructed, vary- 
ing only in unimportant details. This latter instrument I 
will proceed to describe. 
_ A large diagonal prism placed in the meridian inverts the 
image in declination, bringing the north point to the top of 
the field, the stars moving toward the left. Then if I wish to 
map in the eastern sky, I turn the prism to the left upon the 
axis of the telescope, and turn the field-piece with its attach- 
ments the same angle upon its own axis, also to the left, the 
angle being such as will be most convenient to the eye, the 
plane of the field making an angle of about 60° with the hori- 
zontal plane, 
Figure 1 represents the plane of the field of view. In the 
center of the field of view is represented the star-point, by 
which the stars are to be suc- 
cessively occultated, there being 
& projection upon the left, easily 
seen in the absence of illumina- 
tion, to mark the exact point of 
occultation. This star-point de- 
scribes an are, the center of mo- 
tion being represented to the 
el 
of the stars. Into the field piece is fitted a positive eye-piece 
of suitable magnifying power. 
The arc described by the star-point is exactly equal to that 
described by the star-key ; and the levers connecting the two 
are so adjusted that moving the star-key across the width of 
