spot on the object, but in addition that in a wide cone surrounding the straight beam. It is naturally brighter the wider the cone; which explains why in poor light we open the diaphragm of the camera wider than when the light is good; a brightly illuminated object will pass enough light through a narrow opening, but a dimmer object must have as wide an opening as possible in order that enough may get through. The method of condensing the light in a spreading cone so that it shall come back to a point again is by means of a lens; not only is this true of cameras, but also of the eye; in fact everything that has been said thus far about cameras applies perfectly to the eye. There is one thing about the way in which light is brought to a point by a lens that makes the formation of images by this method troublesome in comparison with their formation in pinhole cameras. That is that the cone of light which strikes the lens is condensed as an opposite cone on the other side, and since the formation of an image requires that every point of the object shall be reproduced as a _point_ in the image, there is only one place where the image can come, which is where the tips of the cones of light are. This place is spoken of as the _focus_. Unless the screen or film is exactly there the image will be made up of overlapping circles of light instead of points, and so will be blurred. In a pinhole camera the cones are so small that they cannot overlap, so there is no one place where the image is better than elsewhere; in other words, there is no necessity of focusing. The chief reason that focusing gives trouble is that the farther away from the lens the object is the closer to the lens will be its image; hence if the field of view consists of several objects at different distances they will not all focus at the same level; the distant objects will focus near the lens; the near objects farther back. In practice this trouble is met by using a thick lens, which has a very short focus to begin with, so that a considerable range of distances will be covered without serious blurring, and for finer work by adjusting the distance between the lens and the back of the camera, where the film or plate is, so that there shall be a good focus of the particular object that is desired.