light even of some of the brightest stars must travel hundreds of years before we can see it. And we have seen that there may be other things which might diminish its light on the way, dark nebulae which may screen or stop its light. So that it seems wonderful that we should see stars at all. Nevertheless, we do; we get light from even faint stars across long distances; and that shows there cannot
Fig. 88
be very much in the way between us and them. There may be a little very fine "fog," as we might call it; according to our best information to-day there probably is a little very fine "fog" between us and the most distant stars; but there cannot be much; otherwise we should not get light and even some heat from the stars. The heat that we get is far harder to detect than the light: it has taxed the utmost resources of physicists at the present day to measure it. Apparatus so delicate that it could
