So the RED (or _least_ refracted rays) of all the drops in the position of A,--the YELLOW of those in the position of B,--and the BLUE (or the _most_ refracted rays) of the lowest drops, all meet the eye D, and form a rainbow to the spectator. The reason why the primary bow exhibits the stronger colours is this--because the colours are seen after _one_ reflection and _two_ refractions; but the colours of the secondary (or upper) rainbow undergo _two_ reflections and _three_ refractions.] [Illustration: Here also the _least_ refracted ray is RED, and the _most_ refracted BLUE (as in the former case); but the position of each is reversed.] Q. _Why does a SOAP BUBBLE exhibit such VARIETY of COLOURS?_ A. The changing colour of the bubble depends upon the changing _thickness of the film_ through which the ray passes. Q. _How does the THICKNESS of the FILM affect the COLOUR of the soap bubble?_ A. Because different _degrees of thickness_ produce different _angles of refraction_, and, therefore, different colours reach the eye. Q. _Why is the SOAP BUBBLE so constantly CHANGING its THICKNESS?_ A. As the bubble is _suspended_, the water keeps _running down from the top_ to the bottom of the bubble, till the crown becomes so _thin_ as to burst. Q. _Why are the late EVENING CLOUDS RED?_ A. Because RED rays (being the _least refrangible_) are the _last to disappear_. [Illustration: Here it will be seen that the red ray PA, being reflected on the horizon at A, will be visible to us; but the YELLOW and BLUE rays will be hidden by the curve of the earth.] Q. _Why are the early MORNING clouds RED?_ A. Because RED rays (being the _least refrangible_) are the _first to appear_. _See last figure._--It is evident that PA (the red rays) will be reflected on the horizon before either the yellow or blue ones. Q. _What becomes of the BLUE and YELLOW rays?_ A. They are refracted _below the horizon_, and are soon made invisible by the _curve of the earth_. (_See last figure._) Q. _Why are the EDGES of CLOUDS more LUMINOUS than their CENTRES?_ A. Because the _body of vapour is thinnest_ at the edges of the clouds. Q. _What is the cause of morning and evening TWILIGHT?_ A. When the sun is below the horizon, the rays (which strike upon the atmosphere or clouds) _are bent down towards the earth_, and produce a little light called twilight. _See figure on p. 399._--Here the rays of PA will give _some_ light. Q. _Why is a ray of LIGHT composed of VARIOUS COLOURS?_ A. If solar light were of _one colour only_, all objects would appear of _that one colour_ (or else black.) Q. _Why are some things of ONE COLOUR, and some of ANOTHER?_ A. As every ray of light is composed of all the colours of the rainbow, _some_ things reflect _one of these colours_, and some _another_. Q. _Why do some things reflect ONE COLOUR, and some ANOTHER?_