When viewed from several feet away, the dots coalesce into a halation of amorphous color.

When viewed from several feet away, the dots coalesced into a halation of amorphous color.

In direct portrayal of the sun's corona during total eclipses, photography has offered superior advantages over visual sketching, in the form and exact location of the coronal streamers; but the extraordinary differences of intensity between the inner corona and its outlying extensions are such that halation renders a complete picture on a single plate practically impossible.

In cometary photography, much the same difficulties are present as in photographing the corona: if the plate is exposed long enough to get the faint extensions of the tail, the fine filaments of the coma or head are obliterated by halation and overexposure.

The same mixture makes an excellent backing for dry plates to prevent halation.