Psychological Investigations 219
increased inn range he invented further instruments. Thus we find, in 1889, an "Instrument for testing the perception of differences of Tint'." This might be described as a double wedge photometer, one photometer being set by the examiner and the other by the examinee, who endeavours to match the known tint set by the examiner. Actually Galton got over the expense of wedge photometers by using _ sheets of coloured glass, each rotating on a horizontal wheel on the same axis, and which could thus be set at
*any angle to the examinee's line of sight; a rotation of either wheel caused the light from an illuminated screen to pass through a greater thickness of the coloured glass. For the measurement of white light Galton replaced the sheets of coloured glass by gratings. The whole apparatus was extremely simple; the examinee, with his head screened from the light, looked through - a slit into a horizontal tube blackened inside, at the other end of which were two windows, with outlook on an illuminated screen. Inside the tube in front of the two windows were placed the two wheels carrying the examiner's and the examinee's photometric sheets of glass diametrically, one was controlled by the examiner and the other by the examinee, and the former . recorded the angular settings of both. The difference was a measure of the goodness of the colour matching. The great advantage of the instrument over a wedge _photometer system lies not only in economy, but in the power it gives the experimenter of changing his colours2. Some disadvantage arises from the varying amount of light reflected from sheets at varying angles.
Another instrument exhibited at the same times, but the details of which belong to an earlier period, was a pendulum for "Determining Reaction-Time." This consisted of a fairly massive seconds pendulum, which could be released at an angle of 18' from the vertical ; during its descent it gave a light-signal by brushing against a very light and small mirror which reflected a light off or onto a screen, or on the other hand it gave a soundsignal by a light weight being thrown off the pendulum by impact with a hollow box 4. The position of the pendulum at either of these occurrences is known. The position of the pendulum, when the response is made to the signal, is obtained by means of a thread stretched parallel to the axis of the pendulum by two elastic bands above and below and in a plane perpendicular to that of the motion of the pendulum. This thread moves freely between two parallel bars in a horizontal plane, and pressing a key causes the bars to clamp on the thread, just, for illustration, as the bars of a parallel ruler might close on the thread. This determines the response-position of the pendulum, the motion of which is not suddenly checked by the clamping of the thread, owing to the elastic bands. The horizontal bars are just below
1 Journal of the Anthropological Institute, Vol. xix, pp. 27-29, 1889.
2 I have recently had such a piece of apparatus constructed in the Biometric Laboratory for testing personal equation. Some mechanical difficulty arose in bringing the two coloured windows adequately close together for reasonable comparison. I surmounted this by aid of a prism of Iceland spar, the image of the ordinary ray of one could be juxtaposed to the image of the extraordinary ray of the other, the. other images being cut off by a diaphragm. The slight colouring of the border of the extraordinary .image was found negligible. 3 Ibid. p. 28.
4 A similar pendulum, adjustable to any time of oscillation, in the Galton Laboratory, just touches a delicately-balanced hammer which falls on a bell.