360 ACROPHTSICS. 



rally regarded as primary colours ; but orange is a compound of the 

 two former ; green of the two latter ; and indigo and violet are com- 

 pounds of the two extreme colours, red and blue. The violet rays 

 will deviate most from their original direction ; as they are the most 

 refracted. 



Hence, when a beam of light falls on a simple converging lens, it 

 is not only somewhat scattered at the focus, by the refraction of the 

 spherical surfaces, producing what is called spherical aberration; 

 but the different colours, being unequally refracted, form an image 

 with colored rings : and this effect, called chromatic aberration, 

 long presented the greatest difficulty in forming perfect refracting 

 telescopes. It was remedied by the discovery that flint glass, con- 

 taining lead, forms a longer spectrum than crown glass, for the same 

 deviation ; or, in other words, has a greater dispersive power. 

 Hence, by combining a convex lens of crown glass, with a weaker 

 concave one of flint glass, the latter may counteract the dispersion of 

 the former, without entirely counteracting its refraction ; and this 

 constitutes an achromatic lens, such as is used for the object glass 

 of the best telescopes. 



Light may be decomposed by absorption; as when it meets a 

 blue glass ; which absorbs the other colors, and chiefly transmits and 

 reflects the blue. In this way it may be shown, that most of the 

 colors are compounded; black being the absence of light; and white, 

 a combination, or interpolation of all the colours ; which are sup- 

 posed to be caused by the more or less rapid undulations, or vibra- 

 tions of the ethereal medium. Thus, green glass, or the green leaves 

 of plants present this color, it is supposed, because they reduce the 

 vibrations of the ethereal medium to a certain length or rapidity : 

 these being shortest for the violet, and longest for the red, of all the 

 colors. The formation of colored rings, by thin plates, as in soap 

 bubbles ; or, by diffraction, as around the shadows of small bodies 

 held in a cone of diverging light ; or, by direction, as in the light 

 reflected from a thick concave mirror ; and the colored fringes seen 

 on fibrous, or finely grooved surfaces, are all attributed to the inter- 

 ference of undulations proceeding from different points, and more or 

 less counteracting each other. Of the double refraction and polari- 

 zation of light, we have no room here to speak. 



4. Practical Optics, properly includes the application of op- 

 tical principles to the explanation of natural phenomena, and the con- 

 struction of optical instruments. The rainbow, is formed by rays 

 of light, from the sun or moon, striking drops of water, being re- 

 fracted in entering them, reflected back, in part, from the opposite 

 side of the drops, and refracted again on leaving them, so as to pro- 

 duce prismatic colors, some of which meet the eye. In the inner, 

 or primary bow, the light is refracted downwards, and undergoes but 

 one reflection: while in the outer, or secondary bow, the light 

 striking the lower side of the drop, is first refracted upwards, and 

 reflected twice within the drop, before leaving it : hence, its light is 

 fainter. Solar and lunar halos, or circles of light ; and parhelia, 

 and paraselene or mock suns and moons, are supposed to be caused 

 by the refraction of light through crystals of ice, floating in the air. 



