660 IX. ON EMPIRICAL AND VERBAL STRUCTURES 

jectivity of the instruments and the finite velocity of propagation. When we
discuss the psychological or methodological or semantic significance of science
and scientific method, we deal with different subjects.
When we use the term 'observer' we mean an observer so equipped that he
can do whatever is demanded of him.
What was said about the definition of 'simultaneity' by the aid of the
camera, applies also to ourselves.
The problem of prime importance before us is to find out if 'simultaneity',
as defined, has an 'absolute' and universal significance, or if it is perhaps a
private and relative notion.
We will carry out the analysis in two ways, the first by example, which
will be instructive, though perhaps not completely conclusive; the other, by
the use of the LorentzEinstein transformation.
Let us perform our last experiment, which, with modern methods seems to
be feasible, in a slightly more complicated form.
We can select a dark night in which flashes will photograph well even at considerable distances. We can place powerful projectors at A and B, and we place our camera so that the film will come exactly at C, midway between A and B. We can start the mechanism of the rapidly moving film and, by an electrical contact made at C, we can produce a short flash from each of the two projectors. Because of the assumptions, AC=CB, and equal velocity of the propagation of electrical currents and lightwaves in all directions we shall have by the structural definitions which condition the experiment, one picture in Fig. 2, say at the spot of our moving film marked by 5. The rays of light from A and B would arrive 'simultaneously'  that is, 'at the same time'  and would affect our moving film in one spot. Our definition was for a stationary observer, and under the conditions, the experiment was fairly definite  all the underlying structural assumptions, of course, being taken for granted. Now consider an observer, as shown in Fig. 3, moving uniformly in the
direction from A to B.
Let us assume that he is also equipped with a similar sort of moving picture
camera as the stationary observer, and that just before he passes the point C

