It is found under these conditions that the natural speed made by the eye in passing the 9-cm. opening _ON_ is very well approximated by the pendulum if the latter is allowed to fall through 23.5° of its arc, the complete swing being therefore 47°. The middle point of the pendulum is then found to move from _O_ to _N_ in 110[sigma][19]. If the eye sweeps from _O_ to _N_ in the same time, it will be moving at an angular velocity of 1° in 11.98[sigma] (since the 9 cm. are 9° 11' of eye-movement). This rate is much less than that found by Dodge and Cline (_op. cit._, p. 155), who give the time for an eye-movement of 40° as 99.9[sigma], which is an average of only 2.49[sigma] to the degree. Voluntary eye-movements, like other voluntary movements, can of course be slow or fast according to conditions. After the pendulum has been swinging for some time, so that its amplitude of movement has fallen below the initial 47° and therewith its speed past the middle point has been diminished, the eye in its movements back and forth between the fixation-points can still catch the after-image of _i_ perfectly distinct and not at all horizontally elongated, as it would have to be if eye and pendulum had not moved just together. It appears from this that certain motives are able to retard the rate of voluntary movements of the eye, even when the distance traversed is constant. [19] The speed of the pendulum is measured by attaching a tuning-fork of known vibration-rate to the pendulum, and letting it write on smoked paper as the pendulum swings past the 9-cm. opening. The experiment is now as follows. The room is darkened. Card _T_ is dropped into groove _z_, while _I_ is put in groove _y_ and swings with the pendulum. One eye alone is used. Case 1. The eye is fixed in the direction _EA_. The pendulum is allowed to swing through its 47°. The resulting visual image is shown in Fig. 7:1. Its shape is of course like _T_, Fig. 6, but the part _H_ is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure. Sheets of milk-glass are now dropped into the back groove of _BB_, until the light is so tempered that part _H_ (Fig. 7:1) is _barely but unmistakably_ visible as luminous. The intensity actually used by the writer, relative to that of _EE_, is fairly shown in the figure. (See Plate III.)