1874.] Optical Phenomena of the Atmosphere. 39 
character, and has but little colouring. This form, which is 
very uncommon, appears sometimes when the sky is almost 
clear. 
A few notes on recent appearances of parhelia, in relation 
to the attendant halos, may not be inappropriate. ‘The 
sketch given of blue parhelia accompanying a red halo, 
and clearly external to it, illustrates, I think, a very 
uncommon case. It will also be seen that, while most mock 
suns have an elliptical or oval outline, and the major axis in 
a line with the circumference of the halo, the major axis of 
this lay in a line with the vadius of the halo, and the balls of 
light were not symmetrically placed. 
Cases of parhelia without any halo are very rare. An 
instance may be found recorded in “‘ Nature,” of an oval mock 
sun entirely within the attendant circle, and _ situated 
vertically above the real sun. Most of the cases of parhelia 
noted by me have been in a horizontal line, not a vertical 
one; and those situated above the sun have been very 
deficient both in colouring and definition of form. 
One remarkable feature of these appearances is this, that 
when the parhelia are seen in very perfect form, the halo 
that accompanies them appears only to show one colour, 
and that generally red I believe ; whereas the ordinary solar 
halo has blue for the most part predominant. 
I now pass on to consider the various media, or conditions 
of the atmosphere that give rise to these optical phenomena. 
The form of cloud denominated by Howard cirro-stratus is, 
as he himself remarked, the most frequent originator both 
of lunar and solar halos; but ordinary cirrus also often 
produces them in an imperfect form. The normal or 
‘curling’ type of cirrus seldom shows prismatic tints, 
though we know it to be stri€tly an ice-cloud ; nor does the 
form of cumulus which we may call “snow cumulus.” 
This seems strange when we consider that the halo is 
simply a result of ice refraction; but it may be explained 
upon the hypothesis that, in these two forms of clouds, the 
particles are so massed together, and the prisms and crystals 
so overlaid one upon another, that the refracted rays are 
again combined and a white line produced, as may be seen 
on a close inspection of a fresh snow-flake. 
Taking, therefore, the various forms of cloud, we find 
that halos are usually restricted to the following :— 
I. Cirro-stratus, or as it is sometimes called linear 
cirrus. 
2. Ordinary cirrus. On this form I may remark, that it is 
when somewhat thin and transparent that the halo appears. 
