=357. Images by Small Apertures.=--The straight line movement of light makes possible the _pin-hole_ camera, by which satisfactory photographs have been made. The action of this device may be illustrated by placing a luminous body, a lighted candle, an incandescent lamp, or a gas flame, in front of a piece of cardboard, _S_, which has a small opening in it. Light from the object (see Fig. 349) falls upon a screen, _S_{2}_, so as to produce an _inverted image_. Other applications of this principle will be given later. In Fig. 349 let _PQ_ represent a gas flame, then light from point _P_ at the _top_ of the flame will pass in a straight line through the opening or aperture of the cardboard and strike at _P_{2}_ at the _bottom_ of the illuminated spot upon the screen. Light from _Q_ passing in straight lines through the aperture will strike at _Q_{2}_ at the top of the lighted space. This spot of light will have the same outlines as the luminous body _PQ_ and being formed as just described will be _inverted_. [Illustration: FIG. 349.--Image formed by a small aperture is inverted.] This spot of light, resembling in its outlines the flame, is called an _image_. _An image is defined as an optical counterpart of an object_. Images are formed in a variety of devices, such as _apertures_, _mirrors_, and _lenses_. The _pin-hole camera_ is simply a light-tight box with a small aperture in one side. Light passing through this aperture forms an image upon the opposite side of the interior of the box, of whatever object is in front of the camera. Light entering a room through a _large_ aperture such as a window produces a multitude of overlapping images which blend to form a somewhat evenly illuminated surface. Important Topics 1. Light contrasted with sound (three differences). 2. Bodies: transparent, translucent, opaque. 3. Light: reflected, transmitted, absorbed. 4. Light travels in straight lines, evidence, shadows, umbra, penumbra. 5. Formation of images by small apertures. Exercises 1. Consider the circumference of the earth as 25,000 miles. How many times would the speed of light cover this distance in a second? 2. How soon after any great disturbance takes place on the sun, 93,000,000 miles distant, can it be seen upon the earth? 3. Construct a diagram of the moon's shadow. How much of the sun can one see when in the moon's umbra? When in its penumbra? Have you ever been in either? When? Have you ever been in the earth's umbra? In its penumbra? 4. Explain, using a diagram, the formation of an inverted image by a small aperture. 5. If the sun is 45 degrees above the horizon, what is the height of a pole casting a shadow 60 ft. long? 6. If a shadow 6 ft. long is cast by a 10-ft. pole standing vertically upon a walk, how tall is the tree whose shadow is 42 ft. long, both measurements being made at the same time? 7. Why are the shadows caused by an electric arc lamp so sharply defined?