This property of lenses lays the foundation of the camera obscura, an instrument to be afterwards described. The following principles in relation to images formed by convex lenses may be stated. 1. That _the image subtends the same angle at the centre of the glass as the object itself does_. Were an eye placed at C, the centre of the lens LN, fig. 13, it would see the object OB, and the image IM under the same optical angle, or, in other words, they would appear equally large. For, whenever right lines intersect each other, as OI and BM, the opposite angles are always equal, that is, the angle MCI is equal to the angle OCB. 2. _The length of the image formed by a convex lens, is to the length of the object, as the distance of the image is to the distance of the object from the lens_: that is, MI is to OB :: as CA to CA. Suppose the distance of the object CA from the lens, to be forty-eight inches, the length of the object OB = sixteen inches, and the distance of the image from the lens, six inches, then the length of the image will be found by the following proportion, 48 : 16 :: 6 : 2, that is, the length of the image, in such a case, is two inches. 3. _If the object be at an infinite distance, the image will be formed exactly in the focus._ 4. _If the object be at the same distance from the lens as its focus, the image is removed to an infinite distance on the opposite side_; in other words, the rays will proceed in a _parallel_ direction. On this principle, lamps on the streets are sometimes directed to throw a bright light along a foot-path where it is wanted, when a large convex glass is placed at its focal distance from the burner; and on the same principle, light is thrown to a great distance from lighthouses, either by a very large convex lens of a short focal distance, or by a concave reflector. 5. _If the object be at double the distance of the focus from the glass, the image will also be at double the distance of the focus from the glass._ Thus, if a lens of six inches focal distance be held at twelve inches distance from a candle, the image of the candle will be formed at twelve inches from the glass on the other side. 6. _If the object be a little further from the lens than its focal distance, an image will be formed, at a distance from the object, which will be greater or smaller in proportion to the distance._ For example, if a lens five inches focus, be held at a little more than five inches from a candle, and a wall or screen at five feet six inches distant, receive the image, a large and inverted image of the candle will be depicted, which will be magnified in proportion as the distance of the wall from the candle exceeds the distance of the lens from the candle. Suppose the distance of the lens to be five and a half inches, then the distance of the wall where the image is formed, being twelve times greater, the image of the candle will be magnified twelve times. If MI (fig.