Page:The wonders of optics (1869).djvu/139

Rh the middle at right angles, suffer no change, the others being refracted in proportion to their angles of incidence.



The rays proceeding from the flower cross each other at a certain point: hence the image on the screen is reversed. The dimensions of the image will depend on the distance of the object from the lens. This is a fact we meet with every day, when using an opera-glass or a telescope. Images formed by convex lenses upon a screen are called by opticians real images, in contradistinction to those which are the result of mere reflection, as in the case of plane mirrors. These latter are known as virtual images and are produced by convex lenses as well as by plain reflecting surfaces. In fig. 32, for instance, the unreversed image of the insect seen by the eye is not a real image, but a virtual one,—a fact that might be easily proved by placing a screen in the position of the eye, when it would be found that no image would be formed.

When using an ordinary magnifying-glass we see the virtual image of the object we are looking at, but in the case of a telescope or opera-glass we see the real image of the object, formed by the large lens in front, and