A Guide To The Scientific Knowledge Of Things Familiar Part 87

This muscle is called the "marsupium."

Q. _Why can HAWKS not only see such a long way off, but also objects within half-an-inch of their eye?_

A. Because their eyes are furnished with a broad circular rim which _confines the action of this muscle_, and throws the _cor'nea forward_.

Q. _Into how many PARTS may a RAY of LIGHT be DIVIDED?_

A. Into three parts: BLUE, YELLOW, and RED.

N.B. These 3 colours, by combination, make seven. 1.--RED. 2.--Red and yellow form ORANGE. 3.--YELLOW. 4.--Yellow and blue make GREEN.

5.--BLUE. 6 and 7.--Shades of blue called INDIGO and VIOLET.

Q. _How is it KNOWN, that a ray of light consists of several different colours?_

A. Because, if a ray of light be cast upon a triangular piece of gla.s.s (called a prism), it will be distinctly divided into seven colours: 1.--Red; 2.--Orange; 3.--Yellow; 4.--Green; 5.--Blue; 6.--Indigo; and 7.--Violet.

Q. _Why does a PRISM DIVIDE a ray of light into VARIOUS COLOURS?_

A. Because all these colours have _different refractive powers_. Red is refracted _least_, and blue the _most_; therefore, the _blue_ colour of the ray will be bent to the _top_ of the prism, and the _red_ will remain at the _bottom_.

[Ill.u.s.tration: Here the ray AB received on a prism, would have the blue part bent up to C; the yellow part to D; and the red part no further than E.]

Q. _What is meant by the REFRACTION of a ray?_

A. _Bending it_ from its straight line.

Thus the ray AB of the last figure is refracted at B into three courses, C, D, and E.

Q. _What is the cause of a RAINBOW?_

A. When the clouds opposite the sun _are very dark_, and rain is _still falling_ from them, the rays of the bright sun _are divided by the rain-drops_, as they would be by a prism.

[Ill.u.s.tration: Let A, B, and C be three drops of rain; SA, SB, and SC three rays of the sun. SA is divided into the 3 colours; the blue and yellow are bent _above_ the eye D, and the _red_ enters it.

The ray SB is divided into the three colours; the blue is bent _above_ the eye, and the red falls _below_ the eye D; but the _yellow_ enters it.

The ray SC is also divided into the three colours. The blue (which is bent most) enters the eye; and the other two fall below it. Thus the eye sees the blue of C, and all drops in the position of C; the yellow of B, and of all drops in the position of B; and the red of A, &c.; and thus it sees a rainbow.]

Q. _Does EVERY person see the SAME colours from the SAME DROPS?_

A. No; _no two persons_ see the _same rainbow_.

To another spectator the rays from SB might be _red_ instead of yellow; the ray from SC, yellow; and the blue might be reflected from some drop below C. To a _third_ person the red may issue from a drop above A, and then A would reflect the yellow, and B the blue, and so on.

Q. _Why are there often TWO RAINBOWS at one and the same time?_

A. In _one_ rainbow we see the rays of the sun _entering the rain-drops at the top_, and reflected to the eye _from the bottom_.

In the _other_ rainbow, we see the rays of the sun _entering the rain-drops at the bottom_, and reflected to the top, whence they reach the eye.

[Ill.u.s.tration: Here the ray SA strikes the drop at A,--is refracted or bent to B,--is then reflected to C, where it is refracted again, and reaches the eye of the spectator.]

[Ill.u.s.tration: Here the ray SB strikes the drop at B,--is refracted to A,--is then reflected to C,--is again reflected to D, when it is again refracted or bent till it reaches the eye of the spectator.]

Q. _Why are the COLOURS of the SECOND bow all REVERSED?_

A. Because in _one_ bow we see the rays which enter at the _top_ of the raindrops, _refracted from the bottom_:

But in the _other_ bow we see the rays which enter at the _bottom_ of the raindrops (after two reflections), _refracted from the top_.

[Ill.u.s.tration: Here A, B, C, represent three drops of rain in the PRIMARY (or inner) RAINBOW.

The _least_ refracted line is RED, and BLUE the _most_.

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.]

[Ill.u.s.tration: 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 pa.s.ses.

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_.