Consider now a source of light approaching the slit with the velocity *v*. If *τ'* is the period of the source which now produces a bright line at *D* and Δ*t'* the time interval between departure from the source of two light impulses which now arrive simultaneously at *D*, we evidently have the relation

(4) |

where *c+v*^{[1]} in accordance with the Stewart theory is the velocity of the light before reflection and and are the components which must be added to *c* to give the velocity of light along the paths and after its reflection.

According to the Stewart theory and will be equal to the components in the direction *BD* and *CD* of the velocities of the mirror images of the original source. An idea of the size of these components is most easily obtained graphically. Considering, for example, the point of reflection *C* as a portion of a plane mirror *EF* which is tangent to the concave mirror at *C*, the position of the image can be found by the usual construction, the line connecting source and image being perpendicular to *EF* and the distances *AE* and equal. Both the original source and the image will evidently be moving towards the point *F* with the same velocity *v*. By a similar construction, which has been omitted to avoid confusion, the image produced by reflection from *B* is found to be located as shown, and moves also with the velocity *v* in the direction of the corresponding arrow.

It can be seen from the construction that in the arrangement shown the motion of the image and the corresponding reflected ray *BD* are

- ↑ See note I. p. 138.