In Figure $35-27,$ there are two light sources $($S$1$ and S$2)$ that emit light with a wavelength of $800$ mm at the same amplitude and phase. The point P where the detector is located is on the x$-$axis passing through the light source S$1.$ The phase difference a between the light sources coming from the two light sources and reaching the point P was measured by moving the point P from x$=0$ to x$=+.$ Figure $35-28$ shows the measurement results up to xs $=10$x $10^-7$ m$.$ When reaching $+,$ what is the maximum value of x at which the ray from S$1$ and the ray from S$2$ are exactly in opposite phases at point P$?$