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THREE-DIMENSIONAL MOTION PICTURES Let us consider the line I l f as representing the object; a the small opening in a wall of a darkened room and ^ a screen upon which the image of the object is formed. The point I of the object emits rays in all directions but only the ray / s enters the instrument due to the extreme smallness the dotted line, an image will be formed at this new position, but its size will be found to have varied. It is evident from the figure that the closer the screen is to the opening the smaller the image and vice versa the greater the distance of the screen from the opening the greater is the size of the image. If the screen is kept stationary and the object is set nearer or further away from the opening, the size of the image will vary accordingly and be greater, the closer the object is to the opening. It is quite obvious that, due to the geometrical construction of the figure some definite relation exists between the size of object and image in respect to the distance between them. The ratio between the size of the image and that of the ob ject is called the magnification and is denoted by the letter m, The distances from the object to the opening and from the image to the opening are measured along the axis and are re spectively called the object distance and the image distance, the first being denoted by the letter u and the second by the let ter /. The perfect understanding of these relations between object and image size and distances will prove its value in the study of images formed by lenses and lens systems, The pin-hole opening has been this far considered as small as a theoretical ray of light. In actual practice, such smallness is unattainable so that the opening of a camera obscura always has a certain area and a bundle of rays originated by each object point will enter the camera obscura instead of the single ray previously considered. Another factor has to be taken into consideration in the in vestigation of the size of the pin-hole opening. When a pencil