Why does the pupil become larger in the twilight? So that more rays of light may enter the eye, whereby objects may be more distinctly seen. Why can an owl or a bat see at night? Because the pupil of the eye in them is quite large, and admits much light. Why do they stay in dark places through the day? Because the bright light of day hurts their eyes. May the image of an object be retained in the eye after the eyelids are closed? It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids. Why does a burning coal moved rapidly around, seem like a circle of fire? Because the light from it is retained a short time by the eye, thus seeming to form a complete circle. Why cannot we count the posts in a fence, when riding rapidly in a car? Because the light from each post falls in such quick succession upon the eye, that it cannot distinguish one post from another. Can objects be seen distinctly when placed near the eye? No; when objects are within six inches of the eye they cannot be seen distinctly. Can objects be seen when at a great distance from the eye? Yes; objects may be seen even when millions of miles away from us; thus, the sun is seen although ninety-five millions of miles from the earth, and some of the stars which we behold in the heavens, are still farther away than the sun. Of what use is the eye to us? The eye gives us ideas of the size, the shape, the color, the place, the distance, and the movements of things around us, so that we can use them the better to promote our comfort and our happiness. What may we remember about the light? And God said, “Let there be light;” and there was light. Thus the day was separated from the night, and the sun was made to send forth his silvery rays upon hill and valley, field and forest, causing the rarest buds and most beautiful blossoms to come forth from the lifeless earth, although no eye, save the All-Seeing One, was there to behold them! [Illustration: Locomotive.] [Illustration: Decoration.] CHAPTER III. Heat. Sources of Heat. [Illustration: Illustrated Capital W] What is heat? Heat is that agent which causes the feeling of warmth. Can heat be seen? No; heat cannot be seen, it can only be felt. When we touch a substance hotter than we are, what takes place? A part of the heat from that substance comes to us and causes a feeling of warmth. When we touch a substance colder than we are, what takes place? A part of the heat from us goes to that substance, and we experience a feeling of cold. Does the amount of heat in any substance vary? Yes; water may be heated until it is scalding hot, or the heat may be taken from it until it is frozen into ice. What causes a substance to become cold? Taking away heat from a substance causes it to become cold. Then what is cold?