For the examination of liquids open the prism box and place a few drops upon the ground surface of the lower prism. Close and fasten the prism box when the liquid will be squeezed into a thin film between the prism. Focus the crosswire with the eyepiece, adjust the mirror until the illumination is good, and gently turn the prism box by means of a milled head screw forwards and backwards until a coloured band appears in he field of view. By turning the milled ring at the base of the telescope, a position will be found at which the coloured band will be eliminated, the field of view being partly light and partly dark with a sharp dividing line. Bring this sharp dividing line into coincidence with the intersection of the crosswires and read the refractive index or sugar percentage on the divided arc by means of the magnifier.

The scale is divided to the third decimal place, the fourth being easily oriented by estimation. It will be noted that the hing of the prism box is so constructed that the lower prism in the box may be removed. For the examination of plastic bodies such as resin, remove the lower prism box, soften the material by heat, and smear a little on the exposed surface of the upper prism, then proced as above. To examine crystals or glasses. A specimen should be prepared, similar in shape to the standard glass test piece provided. Moisten the polished surface of the specimen with the smallest possible quantity of monobromonapthalene and place it upon the suface of the prism with the ground edge directed towards the mirror. Care must be taken to avoid excess of monobromonathalne as this may spoil the definition and confuse the reading. When the correct quantity is used, the specimen will adhere firmly to the prism face.The instrument is sent out carefully adjusted, but it is as well to verify the setting by applying the class test piece to the prism and nothing if the reading is that marked on the piece. I this is not the case, adjust the small squared screw at the back of the telescope with the key provided until agreement is secured. The mean of several reading should be taken if great accuracy is desired.