When a sufficient quantity of the clear liquid has passed through the filter, the 200 mm. observation tube is filled with it.  The 100 mm. tube should never be used except in rare cases, when notwithstanding all the means used to effect the proper decolorization of the solution, it is still too dark to polarize in the 200 mm. tube.  In such cases the shorter tube may be used, and its reading multiplied by two.  The zero deviation must then be determined and applied to the product.  This will give the reading which would have been obtained if a 200 mm. tube could have been used, and it only remains to apply the correction determined by the use of the control plate as previously described.

Example: 

Solution reads in 100 mm. tube 47.0
Multiplied by 2 2.0
——­
Product 94.0
Zero reads plus 0.3 0.3
——­
Solution would read in 200 mm. tube 93.7

Reading of control plate 90.4
Sugar value of control plate 90.5
——­
Instrument too low by 0.1
Add 0.1 to 93.7
——­
Correct polarization of solution 93.8

Before filling the tube it must either be thoroughly dried by pushing a plug of filter paper through it, or it must be rinsed several times with the solution itself.  The cover glasses must also be clean and dry, and without serious defects or scratches.  Unnecessary warming of the tube by the hand during filling should be avoided; it is closed at one end with the screw cap and cover glass, and grasped by the other end with the thumb and finger.  The solution is poured into it until its curved surface projects slightly above the opening, the air bubbles allowed time to rise, and the cover glass pushed horizontally over the end of the tube in such a manner that the excess of liquid is carried over the side, leaving the cover glass exactly closing the tube with no air bubbles beneath it, and with no portion of the liquid upon its upper surface.  If this result is not attained, the operation must be repeated, the cover glass being rubbed clean and dry, and the solution again brought up over the end by adding a few more drops.  The cover glass being in position, the tube is closed by screwing on the cap.  The greatest care must be observed in screwing down the caps that they do not press too tightly upon the cover glasses; by such pressure the glasses themselves may become optically active, and cause erroneous readings when placed in the instrument.  It should therefore be ascertained that the rubber washers are in position over the cover glasses, and the caps should be screwed on lightly.  It must also be remembered that a cover glass, once compressed, may part with its acquired optical activity very slowly, and some time must be allowed to elapse before it is used again.