THE ANALYSIS OF LIGHT. 329 



whati3spheri. Lenses are also subject to another imperfec- 

 caiaberraHonr tioD, which is Called splierical abeiratioD. This 

 arises from the fact that the curved surface of a lens is at 

 unequal distances from the object and from the screen 

 which receives the image formed at its focus ; and hence, 

 if one point of the image is perfect, another point is less 

 BO, owing to a difference in the convergence of the rays 

 coming from the center and the edges of the lens. 



Thus, if tlie image is received on a screen of ground glass, it will be found 

 that when the picture is well defined at the center, it will be indistinct at tho 

 edges ; but by bringing the lens nearer the screen, the edges of the imago 

 will be more sharply defined, but the middle is indistinct. To make the im- 

 age perfect, therefore, the marginal portions of the lens should be covered with 

 a circlet of paper, so as to permit those rays only to pass which lie near the 

 axis of the lens. This plan, however, impairs the brightness of the image. 



When tho image formed by the lens is small, the efiect of spherical aberration 

 is scarcely noticed, and by combination of lenses of different refractive powers, 

 it may be ahnost entirely overcome. 



688. The various ravs composino; solar lijrht 



AreaUtherays n i " • , . 



©flight eqnauy are uot ail equally lummous, that is to say, 



brilliant? . , i ■,■,-, .,t 



they do not appear to the eye equally brilliant. 

 The color most visible to the human eye is yellow. 



The luminous intensity of the different colored rays of light may be ex- 

 pressed numerically as follows: — Red, 94; orange, 640; yellow, 1,000; 

 green, 480; blue, 170; indigo, 31; violet, 6.* 



689. According to some authorities, white solar light 

 consists of only three colors — red, yellow and blue, which, 

 by combining, produce the other four colors, orange, 

 green, indigo and violet. 

 w>,.».,»=-,,^o I^ed, vellow, and blue, are, therefore, some- 



>V n at are some- 7*7 7 / y 



times called the times Called the simple colors. 



nmple colors ? i- 



Thus, by the union of red and yellow, we may produce 

 orange ; by yellow and blue, green ; by blue and red, violet ; indigo being 

 considered as merely a shade of blue. Red, yellow, and blue, on the contrary, 

 can not be produced by the mlngUcg of any two other colors. 



When blue and yellow powders are mixed together, blue and yellow rays 

 are reflected to the eye from the minute particles, but the two colors are so 



• It -would appear, from numerous observations, that soldiers are shot during battle 

 according to the color of their dress in the following proportion : — red, VI ; dark green, 7; 

 browTi, 6 ; bluish gray, 5. Eed is therefore the most fatal color, and a light gray tha 

 least so. 



