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 of the analyzing prism, H, which it fits as well, but which must never be touched, as the adjustment of the instrument would be seriously disturbed by moving it.  With the key on the proper nipple it is turned one way or the other until the field is neutral.  Unless the deviation of the zero be greater than 0.5 deg., it will not be necessary to use the key, but only to note the amount of the deviation, and for this purpose the observer must not be content with a single setting, but must perform the operation five or six times, and take the mean of these different readings.  If one or more of the readings show a deviation of more than 0.3 deg. from the general average, they should be rejected as incorrect.  Between each observation the eye should be allowed 10 to 20 seconds of rest.

The “setting” of the zero having been performed as above, the determination of the accurate adjustment of the instrument by means of the “control” quartz plates is proceeded with.  Three such plates will be furnished with each polariscope, which have “sugar values” respectively approximating 80 deg., 90 deg., and 100 deg..  These values may vary with the temperature, and tables are furnished with them which give their exact value at different temperatures, from 10 deg. to 35 deg.  C.