[April 28th 1916]
termingle with each other, and the complicated refraction which the rays of light undergo, while penetrating the layers of different densities, gives to them a distant visibility. It is the gradual growth of this visibility; the gentle progression from a faint unconstancy of the air to a dense, opaque mirage, that I will now endeavour to trace.
We glance out over the desert slowly raising the eye towards the distant mirage. At our feet the air is clear; the intermingling of the strata is here too delicate to make the air apparent to our vision. We carry our glance a little further. We encompass a broader layer of air. We now observe a dim indistinctness with a fine, almost imperceptible tremor. The visibility of the air has commenced. Again we slowly raise our eyes and follow out the shimmer –
*See note on Slip