434 



SPECIAL PHYSIOLOGY. 



of the light passing through it; the refractive power of combustible 

 bodies is, however, greater than their density would indicate. The 

 spreading or dispersion of the white solar beam by a prism, into the 

 colored spectrum, already described (p. 431), is explained by assuming 

 that its different colored rays, have different degrees of ref Tangibility. 

 The violet, or most rapidly undulating rays, are most bent out of their 

 straight course, whilst the red, or more slowly undulating rays, are the 

 least easily refracted or turned aside. 



When parallel rays fall directly upon a double convex glass lens, such 

 as a common pocket-lens, i. e., upon a refracting medium having two 

 spherically convex surfaces, the ray coinciding with its axis passes 

 through, unchanged in direction, without undergoing any refraction ; 

 all the other rays, however, are twice refracted, first, on entering, 

 towards a perpendicular to their point of entrance into the lens, and 

 then, on issuing, from a perpendicular to their point of exit from the 

 lens. These refracted rays, through whatever part of the lens they 

 pass, meet the central rays at a certain point, called the principal focus 

 of the lens; the distance of this from the lens, is called its proper focal 

 distance, and is determined by the degree of convexity and the refrac- 

 tive power of the lens. As a lens acts either way, it has two principal 

 foci, one opposite the centre of each surface. When the rays of light 

 proceed from a radiant point, situated in one principal focus of a lens, 

 and pass through the lens, the emergent rays are parallel, just as 

 parallel rays converge to the principal focus. When, however, the 

 radiant point is further from the lens than its principal focus, but not 

 so remote that the rays issuing from it enter the lens in parallel lines, 

 then the rays converge to a point or focus, which is nearer the lens 

 the greater the distance of the radiant po^nt from its principal focus. 



When an object (Diagram I) a, b, c, is placed in front of a lens, I, 

 so that the rays of light emitted from its several points, diverge as 

 they fall upon the refracting surface, those which proceed from the 

 central point of the object, 5, form a conical pencil of rays, called a 



Diagram I. 



Diagram I. Diagram, illustrating the formation of an inverted image of an object in the focus of a dou- 

 ble convex glass lens. Z, the lens, seen edjrewnys. a, fc, c, an arrow, representing the object, a', //, c', the 

 inverted image of the same, a, pencil of rays, from the point of the arrow, refracted on entering, and 

 emerging from, the lens, to meet in the point a', c, another pencil of rays, from the opposite end of the 

 arrow, acted on in a similar manner, and converging at the point c'. In order to avoid confusion in the 

 diagram, only the central ray of the pencil from the point b is here shown; it alone undergoes no refraction. 



direct pencil ; all the divergent rays of this pencil, after having under- 

 gone refraction, converge on the other side of the lens, meeting the 

 central ray, which has passed through without undergoing any change 

 in direction, at, or near, a common focus, b'. The rays from all the 

 other points of the object form more or less oblique pencils. Those 



