FOR many purposes, both in physics and engineering, the derivative of a graphical record has to be taken with all the accuracy that the record allows. For this purpose, either a numerical method of differencing is usually resorted to, or a mechanical process of laying a tangent. Perhaps the best known of this latter type is that which depends on the principle that a curve and its reflection in a piano mirror pass continuously into one another when the reflecting surface lies along the normal. A prismatic derivator has been designed by v. Harbou and has been produced by Askania-Werke A.G., Bambergwerk, Berlin-Friede-nan, which avoids the difficulty usually experienced of recognising when a curve and its reflection are continuous. In this new instrument a right-angled prism is placed with its hypotenuse on the paper and with the intersection of the opposite edge approximately along the normal to the curve. The portions of the curve on each side of the point at which the tangent is required, as seen after refraction through the prism faces, will not join up at the prism edge unless that edge is accurately along the normal. From the published description of the instrument, it appears that there is a number of other adjuncts that tend to increase the accuracy and effectiveness of the apparatus.