REFRACTION IN THE EYE 669 



The different calculations of physicists in regard to the velocity of 

 light have been remarkably uniform in their results. The lowest calcu- 

 lations put it at about 185,000 miles (297,725 kilometers) in a second, 

 and the highest, at about 195,000 miles (313,818 kilometers). The rate 

 of propagation usually is assumed to be about 192,000 miles (309,000 

 kilometers). 



The intensity of light is in proportion to the amplitude of the vibra- 

 tions. The intensity diminishes in inverse ratio to the square of the 

 distance. 



In the theory of the colors into which white light may be decomposed 

 by prisms, it is a matter of demonstration that the waves of the different 

 colors are not of the same length. The decomposition of light is due to 

 differences in the refrangibility of the different colored rays as they pass 

 at an angle through a medium denser than the air. 



The analysis of white light into the colors of the spectrum shows 

 that it is compound; and by synthesis, the colored rays may be 

 brought together, producing white light. Colors may be obtained by 

 decomposition of light by transparent bodies, the different colored rays 

 being refracted, or bent, by a prism, at different angles. It is not in 

 this way, however, that the colors of different objects are produced. 

 Certain objects have the property of reflecting the rays of light. A 

 smooth polished surface, like a mirror, may reflect all the rays ; and the 

 object then has no color, only the reflected light being appreciated by 

 the eye. Certain other objects do not reflect all the rays of light, some 

 of them being lost to view, or absorbed. When an object absorbs all the 

 rays it has no color and is called black. When an object absorbs the 

 rays equally and reflects a portion of these rays without decomposition, 

 it is gray or white. There are many objects, however, that decompose 

 white light, absorbing certain rays of the spectrum and reflecting others. 

 The rays not absorbed, but returned to the eye by reflection, give color 

 to the object. Thus, if an object absorbs all the rays of the spectrum 

 except the red, the red rays strike the eye, and the color of the object is 

 red. So it is with objects of different shades, the colors of which are 

 given by the unabsorbed rays. 



A mixture of different colors in certain proportions will result in 

 white. Two colors, which, when mixed, result in white, are called com- 

 plementary. The following colors of the spectrum bear such a relation 

 to each other : Red and greenish blue ; orange and cyanogen-blue ; 

 yellow and indigo-blue ; greenish yellow and violet. 



The fact that impressions made on the retina persist for an appre- 

 ciable length of time affords an illustration of the law of complementary 

 colors. If a disk, presenting divisions with two complementary colors, is 



