As one of the most important results of what we heard in these days I consider the density law of interplanetary dust derived from zodiacal light observations by the deep space probes going out to Jupiter and going in to 0.3 AU. The dependence on the distance to the sun R seems to be nearly as R-1. This finding is in agreement with a new discussion of ground based observations which was reported by Dumont. The density law was one of the open questions for a long time; for me this represents a break-through.