2ndly--The nerves (which receive the impression) have _one point of union_, before they reach the brain. Q. _Why do we SEE OURSELVES in a GLASS?_ A. The rays of light from our face _strike against the surface of the glass_, and (instead of being absorbed) _are reflected_, or sent back again to our eye. Q. _Why are the rays of light REFLECTED by a MIRROR?_ A. Because they cannot _pass through the impenetrable metal_ with which the back of the glass is covered; so they _rebound back_, just as a _marble_ would do if it struck against a wall. Q. _When a marble is rolled towards a wall, what is that path THROUGH WHICH IT RUNS called?_ A. The line of the _angle of incidence_. Q. _When a marble REBOUNDS back again, what is the path it THEN describes called?_ A. The line of the _angle of reflection_. Q. _When the light of our face goes TO the GLASS, what is the path through which it goes CALLED?_ A. The line of the _angle of incidence_. Q. _When the light of our face is reflected BACK again from the mirror, what is this RETURNING path called?_ A. The line of the _angle of reflection_. Q. _Why does our reflection in a mirror seem to APPROACH us as we walk TOWARDS it, and to RETIRE FROM us as WE retire?_ A. Because the line _of the angle of incidence_ is always _equal_ to the _line and angle of reflection_. [Illustration: Here CA, EA and DB, FB are the lines of the angle of incidence; and GA, KA and HB, LB are the lines of the angle of reflection. When the arrow is at CD, its shadow will appear at GH, because the line CA=GA and the angle CAB=angle GAB, &c.; and the same may be said about the point D.] Q. _Why can a man see his WHOLE PERSON reflected in a LITTLE MIRROR not 6 inches in length?_ A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_. Take the last figure--CD is much larger than the mirror AB; but the head of the arrow C is reflected obliquely behind the mirror to G; and the barb D appears at H.--Why? Because the line CA=AG and the angle CAB=angle GAB, &c. The same may be said of the point D. Q. _Why does a SHADOW in WATER always appear TOPSY-TURVY?_ A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_. [Illustration: Here the arrow-head A strikes the water at F, and is reflected to D; and the barb B strikes the water at E, and is reflected to C. If a spectator stands at G, he will see the reflected lines CE and DF, produced as far as G. It is very plain that the more elevated object A will strike the water, and be projected from it more perpendicularly than the point B, and therefore the shadow will seem inverted.] Q. _When we see our SHADOW in WATER, why do we seem to STAND on our HEAD?_ A. Because the _line of the angle of incidence_ is always equal to the _line and angle of reflection_.