The _radiant point_ is that _from which diverging rays of light are emitted_. 463. _What is the focus?_ The _focus_ is the point to which _converging rays are directed_. _Diverging_, starting from a point, and separating. _Converging_, drawing together towards a point. 464. _What is the constitution of a ray of light?_ A ray of _white light_, as we receive it from the sun, is composed of _a number of elementary rays_, which, with the aid of a triangular piece of glass, called a _prism_, may be separated, and will produce under refraction the following colours:-- 1. An _extreme red_ ray--a mixture of _red_ and _blue_, the red _predominating_. 2. _Red._ 3. _Orange_--red passing into and combining with yellow. 4. _Yellow_--the most luminous of all the rays. 5. _Green_--yellow passing into and combining with the blue. 6. _Blue._ 7. _Indigo_--a dark and intense blue. 8. _Violet_--blue mingled with red. 9. _Lavender grey_--a neutral tint. 10. Rays called _fluorescent_, which are either of a _pure silvery blue_, or a _delicate green_. 465. _Why is a ray of light, which contains these elementary rays, white?_ Because the colour of light is governed by the _rapidity of the vibrations of the ether-waves_. When a ray of light is refracted by, or transmitted through a body, its _vibrations are frequently disturbed and altered_, and thus a _different impression_ is made upon the _eye_. Light which gives 37,640 vibrations in _an inch_, or 458,000,000,000,000 in a _second of time_, produces that sensation upon the eye which makes the object that directs the vibrations appear _red_. _Yellow_ light requires 44,000 vibrations _in an inch_, and 535,000,000,000,000 in a _second of time_. And the other colours enumerated (_see_ 464) all require different _velocities of vibration_ to produce the colours by which they are distinguished. [Verse: "The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."--MATT. V.]