Let I be the point of maximum illumination of the circle, and therefore E the point of maximum enlightenment of the triangle. (E of course varying perversely as the square of the distance from O.)

Let WH be fixed absolutely, and remain always in contact with the circle, and let the direction of OI be also fixed.

Now, so long as WEG preserves a perfectly straight course, GH cannot possibly come into contact with the circle, but if the force of illumination, acting along OI, cause it to bend (as in Fig. 2), a partial revolution on the part of WEG and GH is effected, WEG ceases to touch the circle, and GH is immediately brought into contact with it. Q.E.F.

The theory involved in the foregoing Proposition is at present much controverted, and its supporters are called upon to show what is the fixed *point,* or '*locus standi,*' on which they propose to effect the necessary revolution. To
make this clear, we must go to the original Greek, and remind our readers that the true point or 'locus standi' is in this case ἄρδις, (or