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. The three different appearances presented by the field are best shown in fig. 2.

ART. 1399. With the milled head set at the point which gives the appearance of the middle disk, as shown above, the eye of the observer is raised to the reading tube K, which is adjusted to secure a plain reading of the divisions, 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 position 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.

ART. 1400. 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 a small 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 II, 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° 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 0.2° from the general average, they should be rejected as incorrect. Between each observation the eye should be allowed a moment 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. One or more such plates will be furnished with each polariscope, and as their sugar values vary with changes of temperature, each plate will be accompanied by a table giving its exact value at different temperatures from 10° to 35° C.

ART. 1401. The plate is placed in the instrument and the field observed; it will be seen that the uniform appearance of the field is changed. The milled head is turned to the right until the exact point of neutrality is reestablished, just as described above in setting the zero. The scale is read, the observation repeated, the reading taken again, and so on until five or six readings have been made. The average is taken, readings being rejected which show a divergence of more than 0.2, and the result corrected for the deviation of the zero point, if any was found, the deviation being added if it was to the left and subtracted if to the right. If the adjustment of the instrument be correct, the result should be the value of the control plate used, as ascertained from the table, for the temperature of 20° C. A variation of 0.2 from the established values may be allowed for errors of observation, temperature, etc., but in the hands of a careful observer a deviation greater than this, after a careful setting of the zero, shows that the instrument is not accurately adjusted.

ART. 1402. After the adjustment of the instrument the setting of the zero point is dispensed with, the indication of the scale for sugar solutions being corrected by the amount of deviation shown in the reading of the control plate from its established value, as ascertained from the table, at the temperature of the room.

For example: A sugar solution polarizes 80.5; the control plate just before had given a polarization of 91.4, the temperature of the room during both observations being 25° C. According to the table the value of the control plate at 25° C. is 91.7; the reading is therefore 0.3 too low, and 0.3 is added to the reading of the sugar solution,

making the correct result 80.8. The temperature of the room must be ascertained from a standardized thermometer placed closed to the instrument and in such a position as to be subject to the same conditions.

The use of the new apparatus of Schmidt & Haensch, known as the triple-shadow polariscope, is also permitted and recommended. The accompanying diagram (fig. 3) shows the appearance of the field of this instrument in the three positions of the quartz wedge.

Preparation of the sugar solution for polarization.

ART. 1403. If the sample is not entirely uniform, it must be thoroughly mixed before weighing, after all the lumps are broken up, best with a mortar and pestle. Then 26.048 grams are weighed on the balance in the tared German-silver dish furnished for this purpose. Care must be taken that the operations for mixing and weighing are not unduly prolonged, otherwise the sample may easily suffer considerable loss of moisture, especially in a warm room. The weighed portion of sugar is washed by means of a jet from a wash bottle into a 100-cubic-centimeter flask, the dish being well rinsed three or four times and the rinsings added to the contents of the flask. The water used must be either distilled water or clear water which has been found to have no optical activity. After the dish has been thoroughly rinsed, enough water is added to bring the contents of the flask to about 80 cubic centimeters, and it is gently rotated until all the sugar has dissolved. The flask should be held by the neck with the thumb and finger and the bulb not handled during this operation. Care must be taken that no particle of the sugar or solution is lost. To determine if all the sugar is dissolved, the flask is held above the level of the eye, in which position any undissolved crystals can be easily seen at the bottom. The character of the solution is now observed. If it be colorless or of a very light straw color and not opalescent, so that it will give a clear, transparent liquid on filtration through paper, the volume is made up directly with water to the 100-cubic-centimeter mark on the flask. Most sugar solutions, however, will require the addition of a clarifying or decolorizing agent, in order to render them sufficiently clear and colorless to polarize. In such case, before making up to the mark, a solution of subacetate of lead is added, which is prepared as follows: Boil an aqueous solution of lead acetate with an excess of