UNEQUAL REFRAXGIBILITY OF DIFFERENT COLORS HI 



or less distorted. A straight rod partly immersed in water, as FD', appears 

 sharply bent at the surface, and slightly curved beneath, assuming the appnreut 

 direction OD". Moreover, though, as in this case, the real direction should 

 pass through the eye, so that in a uniform medium, only the extremity could be 

 visible, the effect of refraction gives a lateral view of all th(; part immersed. 



The next important step in the progress of optical discovery, after the detec- 

 tion of the general law of refraction, was made by the illustrious Newton, who, 

 in the year 1672, communicated to the Royal Society the experimental resean-hes 

 by which he established the compound nature of light, and the unequal refran- 

 gibility of its component rays, lie held that the common white light of the 

 sun is made up of elementary rays differing at the same time in color and in 

 refrangibility. The number of tints vvhich he considered sufficiently distinct 

 to be regarded as independent components is seven. It seems unnecessary, 

 however, to suppose the existence of more than three elementary colors, it being 

 possible, by mingling these in various proportions, to produce all the rest, while 

 the degrees of refrangibility between the extreme limits vary through an infi- 

 nite number of infinitely small differences. 



Newton's method of demonstrating the truth of his doctrine was as simple 

 as it is ingenious. The colors which border the images of objects observed 

 through prisms of glass or other transparent substances, or through cylindrical 

 or globular vessels filled with water, had long been familiar. Newton placed 

 such a prism in the path of a ray of the sun's light, introduced through a small 

 aperture into a dark room, and received the refracted image or spectrum upon a 

 white screen placed at some distance. Before the interposition of the prism 

 the beam produced upon the screen a white and circular image of the sun itself. 

 But after the rays had been bent by refraction the image appeared very much 

 elongated in the direction of the refraction, and brilliantly colored in a series of 

 tints, passing by insensible gradations from red, through orange, ydlow, green, 

 blue, and indigo to violet. This last color was at the end most refracted. In 

 turning the prism around an axis parallel to its edges, Newton observed that 

 the deviation of the spectrum from the original direction of the sun's rays was 

 variable, increasing from a certain minimum (experimentally found) by turuino- 

 the prism either to the right or to the left. This minimum corresponds to that 

 particular position of the prism at which the angles of incidence and emergence 

 are equal. Upon this observation he founded a test experiment in regard to the 

 refrangibility of the rays of different colors. ^Making a small circular aperture 

 in the screen upon which the spectrum was formed, at a point where, by turn- 

 ing the prism, he could pass tlie entire spectrum over it, he placed behind the 

 aperture a second prism, which thus received, successively, rays of a single 

 color only. At a distance behind the second prism a second screen intercepted 

 the light which passed through it, when it was observed that this second image, 

 instead of being elongated like the first, remained sensibly circular, while the 

 positions of the circles of different colors upon the screen w(!re further and 

 further removed from the original direction of the unrefractcd rays as the tints 

 a-ceuded from red to violet. This phenomenon of the separation of the compo- 

 nent colors of light by refraction has been called dispersion. Newton was of 

 opinion that the dispersive powers of all bodies are equal; or, in other words, 

 proportional to their refractive powers; and that, the mean refractive; powers of 

 two bodies being equal, their refractive powers for each particular color must be 

 equal also. Both these suppositions, as we shall see, are unfounded. 



The discovery of Newton furnished an easy explanation of the interesting- 

 natural phenomenon of the rainbow. This beautiful meteor had been the sub- 

 ject of many unsatisfactory speculations; and though rfc X>w«//^/.y, as early as 

 IGll, had furnished a true theory of the manner of formation of the inner bow, 

 he had not been able to account for its colors. He showed that therci is a certain 

 incidence at which, if the parallel rays of the sun fall upon the anterior surface 



