=401. The Achromatic Lens.=--The existence of these different kinds of glass makes possible a combination of lenses in which dispersion is entirely overcome with the loss of only about one-half of the refraction. Such a combination is shown in Fig. 403. It is called an _achromatic lens_, since images formed by it are not colored but white (_a_ = without, _chroma_ = color). _The achromatic lens consists of a double convex lens of crown glass combined with a plano-concave lens of flint glass._ Achromatic lenses are used in all high-grade optical instruments such as telescopes and microscopes. The colored images that are sometimes seen in cheap opera glasses show the result of not using achromatic lenses. [Illustration: FIG. 403.--An achromatic lens. _C_ is of crown glass; _F_, of flint glass.] =402. The Color of Bodies.=--Project the spectrum of sunlight upon a white surface in a darkened room. Now place in different parts of the spectrum objects of various colors. Red objects will show brilliant red when at the red end of the spectrum but look black at the blue end, while blue objects appear blue only at the blue end. These facts indicate that the color of an object depends upon two things: (a) _the light that falls upon it and_ (b) _the light which it sends to the eye_. A _black_ surface absorbs all color while a _white_ one reflects all wave lengths to the eye in the same proportion that they come to it. A white object will appear red in red light, and blue in blue light since it reflects both of these. A _colored_ object reflects light of its own color but absorbs all others. The color then of a body is due to the light which it does not absorb, but which comes from it to the eye. _403. The color of transparent bodies_, such as colored glass, is due to the presence of a _dye_ or _pigment_ contained in the body. This pigment absorbs a part of the light, the part transmitted giving the color. This may be shown by holding a sheet of colored glass in a beam of light either before or after it has passed through a prism. Some colors, as red, may be found to be nearly _pure_, only the red passing through, while green glass often transmits in addition to the green some yellow and some red light.