A. It is _so far off_, that we cannot distinguish any difference between the _length of the rays_ which issue from the _edge_, and those which issue from the _centre_. [Illustration: The rays AD and CD appear to be no longer than the ray BD; but if all the rays seem of the same length, the part B will not seem to be nearer to us than A and C, and therefore ABC will look like a flat or straight line. The rays AD and CD are 240,000 miles long. The ray BD is 238,910 miles long.] Q. _Why do the SUN and STARS (which are spheres) appear to be FLAT surfaces?_ A. Because they are such an _immense way off_, that we can discern _no difference of length_ between the rays which issue from the _edge_, and those which issue from the _centre_ of these bodies. The rays AD and CD appear no longer than BD; and as B appears to be no nearer than A or C, therefore ABC must all seem equally distant; and ABC will seem a flat or straight line. (See last figure.) Q. _Why does DISTANCE make an object INVISIBLE?_ A. Because the angle (made by the _perpendicular height_ of the distant object _with our eye_) is so very _acute_, that _one_ line of the angle _merges in the other_. [Illustration: Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck.] Q. _Why do TELESCOPES enable us to SEE objects INVISIBLE to the naked eye?_ A. Because they _concentrate several rays_ within the tube of the telescope, and _bend them upon the mirror or lens_, which acts as a _magnifying glass_. Q. _When a SHIP (out at sea) is approaching the shore, why do we SEE the small MASTS before we see the bulky HULL?_ A. Because the _earth is round_, and the _curve_ of the sea _hides the hull_ from our eyes, after the tall _masts_ have become visible. [Illustration: Here only that part of the ship above the line AC can be seen by the spectator A; the rest of the ship is hidden by the swell of the curve DE.] Q. _What is meant by REFRACTION?_ A. The _bending of a ray of light_, as it passes from one medium to another. Q. _How is a ray of light BENT, as it passes from one medium to another?_ A. When a ray of light passes into a _denser_ medium, it is bent _towards the perpendicular_. When it passes into a _rarer_ medium, it is bent _from_ the perpendicular. [Illustration: Suppose DE to be a perpendicular line. If AB (a ray of light,) enters the water, it will be bent _towards_ the perpendicular to C. If (on the other hand) CB (a ray of light) emerges _from_ the water, it would be bent _away from_ the perpendicular towards A.] Q. _Why does a SPOON (in a glass of water) always appear BENT?_ A. Because as the light of the spoon _emerges from the water_, it is _refracted_. And the spoon looks like ABC. (See the last figure.)