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involved in the notion of figure。 But since something must be
reflected in them and figure cannot be； it remains that colour alone
should be reflected。 The colour of a bright object sometimes appears
bright in the reflection； but it sometimes； either owing to the
admixture of the colour of the mirror or to weakness of sight； gives
rise to the appearance of another colour。
However； we must accept the account we have given of these things in
the theory of sensation； and take some things for granted while we
explain others。
3
Let us begin by explaining the shape of the halo； why it is a circle
and why it appears round the sun or the moon or one of the other
stars： the explanation being in all these cases the same。
Sight is reflected in this way when air and vapour are condensed
into a cloud and the condensed matter is uniform and consists of small
parts。 Hence in itself it is a sign of rain； but if it fades away；
of fine weather； if it is broken up； of wind。 For if it does not
fade away and is not broken up but is allowed to attain its normal
state； it is naturally a sign of rain since it shows that a process of
condensation is proceeding which must； when it is carried to an end；
result in rain。 For the same reason these haloes are the darkest。 It
is a sign of wind when it is broken up because its breaking up is
due to a wind which exists there but has not reached us。 This view
finds support in the fact that the wind blows from the quarter in
which the main division appears in the halo。 Its fading away is a sign
of fine weather because if the air is not yet in a state to get the
better of the heat it contains and proceed to condense into water；
this shows that the moist vapour has not yet separated from the dry
and firelike exhalation： and this is the cause of fine weather。
So much for the atmospheric conditions under which the reflection
takes place。 The reflection is from the mist that forms round the
sun or the moon； and that is why the halo is not seen opposite the sun
like the rainbow。
Since the reflection takes place in the same way from every point
the result is necessarily a circle or a segment of a circle： for if
the lines start from the same point and end at the same point and
are equal； the points where they form an angle will always lie on a
circle。
Let AGB and AZB and ADB be lines each of which goes from the point A
to the point B and forms an angle。 Let the lines AG； AZ； AD be equal
and those at B； GB； ZB； DB equal too。 （See diagram。）
Draw the line AEB。 Then the triangles are equal； for their base
AEB is equal。 Draw perpendiculars to AEB from the angles； GE from G；
ZE from Z； DE from D。 Then these perpendiculars are equal； being in
equal triangles。 And they are all in one plane； being all at right
angles to AEB and meeting at a single point E。 So if you draw the line
it will be a circle and E its centre。 Now B is the sun； A the eye； and
the circumference passing through the points GZD the cloud from
which the line of sight is reflected to the sun。
The mirrors must be thought of as contiguous： each of them is too
small to be visible； but their contiguity makes the whole made up of
them all to seem one。 The bright band is the sun； which is seen as a
circle； appearing successively in each of the mirrors as a point
indivisible to sense。 The band of cloud next to it is black； its
colour being intensified by contrast with the brightness of the
halo。 The halo is formed rather near the earth because that is calmer：
for where there is wind it is clear that no halo can maintain its
position。
Haloes are commoner round the moon because the greater heat of the
sun dissolves the condensations of the air more rapidly。
Haloes are formed round stars for the same reasons； but they are not
prognostic in the same way because the condensation they imply is so
insignificant as to be barren。
4
We have already stated that the rainbow is a reflection： we have now
to explain what sort of reflection it is； to describe its various
concomitants； and to assign their causes。
Sight is reflected from all smooth surfaces； such as are air and
water among others。 Air must be condensed if it is to act as a mirror；
though it often gives a reflection even uncondensed when the sight
is weak。 Such was the case of a man whose sight was faint and
indistinct。 He always saw an image in front of him and facing him as
he walked。 This was because his sight was reflected back to him。 Its
morbid condition made it so weak and delicate that the air close by
acted as a mirror； just as distant and condensed air normally does；
and his sight could not push it back。 So promontories in the sea
'loom' when there is a south…east wind； and everything seems bigger；
and in a mist； too； things seem bigger： so； too； the sun and the stars
seem bigger when rising and setting than on the meridian。 But things
are best reflected from water； and even in process of formation it
is a better mirror than air； for each of the particles； the union of
which constitutes a raindrop； is necessarily a better mirror than
mist。 Now it is obvious and has already been stated that a mirror of
this kind renders the colour of an object only； but not its shape。
Hence it follows that when it is on the point of raining and the air
in the clouds is in process of forming into raindrops but the rain
is not yet actually there； if the sun is opposite； or any other object
bright enough to make the cloud a mirror and cause the sight to be
reflected to the object then the reflection must render the colour
of the object without its shape。 Since each of the mirrors is so small
as to be invisible and what we see is the continuous magnitude made up
of them all； the reflection necessarily gives us a continuous
magnitude made up of one colour； each of the mirrors contributing
the same colour to the whole。 We may deduce that since these
conditions are realizable there will be an appearance due to
reflection whenever the sun and the cloud are related in the way
described and we are between them。 But these are just the conditions
under which the rainbow appears。 So it is clear that the rainbow is
a reflection of sight to the sun。
So the rainbow always appears opposite the sun whereas the halo is
round it。 They are both reflections； but the rainbow is
distinguished by the variety of its colours。 The reflection in the one
case is from water which is dark and from a distance； in the other
from air which is nearer and lighter in colour。 White light through
a dark medium or on a dark surface （it makes no difference） looks red。
We know how red the flame of green wood is： this is because so much
s