e tube J. The telescope is moved in or
out until the proper focus is secured, so as to give a clearly defined
image, when the field of the instrument will appear as a round,
luminous disk, divided into two halves by a vertical line passing
through the center, and darker on one half of the disk than on the
other. If the observer, still looking through the telescope, will now
grasp the milled head M and rotate it, first one way and then the
other, he will find that the appearance of the field changes, and at a
certain point the dark half becomes light, and the light half dark. By
rotating the milled head delicately backward and forward over this
point he will be able to find the exact position of the quartz wedge
operated by it, in which the field is neutral, or of the same
intensity of light on both halves.

[Illustration]

The three different appearances presented by the field are best shown
in the above diagram. With the milled head set at the point which
gives the appearance of the middle disk as shown, the eye of the
observer is raised to the reading tube, K, and the position of the
scale is noted. It will be seen that the scale proper is attached to
the quartz wedge, which is moved by the milled head, and attached to
the other quartz wedge is a small scale called a vernier which is
fixed, and which serves for the exact determination of the movable
scale with reference to it. On each side of the zero line of the
vernier a space corresponding to nine divisions of the movable scale
is divided into ten equal parts. By this device the fractional part of
a degree indicated by the position of the zero line is ascertained in
tenths; it is only necessary to count from zero, until a line is found
which makes a continuous line with one on the movable scale.

With the neutral field as indicated above, the zero of the movable
scale should correspond closely with the zero of the vernier unless
the zero point is out of adjustment.

If the observer desires to secure an exact adjustment of the zero of
the scale, or in any case if the latter deviates more than one-half of
a degree, the zero lines are made to coincide by moving the milled
head and securing a neutral field at this point by means of the small
key which comes with the instrument, and which fits into a nipple on
the left hand side of F, the fixed quartz wedge of the compensating
system. This nipple must not be confounded with a similar nipple on
the right hand side o