VERNIER, a device for reading scales, in vented by Pierre Vernier (q.v.). It consists of an auxiliary scale of n intervals sliding in contact with the scale to be read, and occupying therein a space of n 1 intervals. Each inter val of the vernier scale will then occupy 1 intervals on the main scale. Let us assume that the main scaie reads from right to left, and the vernier scale from left to right, and that 10 intervals in the vernier scale occupy the place of 11 intervals on the main scale. Let the 0 of the vernier scale be 12.8 intervals to the left of the 0 of the main scale. It will then be situated between 12 and 13 of the main scale. The one of the vernier scale will be at 12.8-1.1 on the main scale, the two at 12.8-2.2, and finally. the 8 at 12.8-8.8=-.4 on the main scale. That is, the figure 8 on the vernier will coincide with a graduation on the main scale. We thus see how to read to tenths of an interval: the in tegral part of the number of units by which the O's of the two scales differ is read in the usual manner from the main scale., while the number of tenths is read by determining which index on the vernier scale most nearly coincides with an index on the main scale. This type of
vernier is known as the retrograde or reverse vernier. In the direct vernier, vernier and main scale read in the same direction, and the vernier intervals are 1— intervals on the main scale. If the distance between the O's is a the dis tance between the 0 of the main scale and c on the vernier scale is a + c (1 — c c — .When b•=c., this is integral, and the reading b on the vernier scale coincides with a reading on the main scale. Verniers are used for reading distances and angles, and the principle has been applied to a form of chronosocope used in psychological experi ments concerning the reaction-time. Here the main scale is represented by a pendulum mak ing n oscillations per minute, the vernier by a pendulum making n I oscillations per minute, and the interval between the times at which the two pendulums are released is measured by the number of oscillations it takes for them to swing in unison. See DIAGONAL SC.ALE ; PSY CHOLOGICAL APPARATUS. Consult Ludlow, (Subscales, Including Verniers> (Von Nos trand's Engineering Magazine, New York 1882).