DIOPTRICS OF THE EYE. 289 



prolongation of this line cuts the secondary axis, A-o, marks the 

 conjugate focus of A and gives the position at which all of the 

 rays will be focused to form the image, a. In calculating the 

 position of the image of any object in front of the lens the same 

 method may be followed, the construction being drawn to de- 

 termine the images for two or more limiting points, as shown 

 in Fig. 118. Let A-B be an arrow in front of the lens. The image 

 of A will be formed at a on the secondary axis A-o, and the image of 

 B at 6 along the secondary axis B-o. The images of the intervening 

 points will, of course, lie between a and b; so that the image of the 

 entire object will be that of an inverted arrow. This image may be 

 caught on a screen at the distance indicated by the construction if 

 the latter is drawn to scale. The principal focus of a convex lens 

 may be determined experimentally or it may be calculated from the 

 formula ~ - + ~ = y, in which / represents the principal focal dis- 

 tance and p and p l , the conjugate foci for an object farther away 

 than the principal focal distance. That is, if the distance of the 

 object from the lens, p, is known, and the distance of its image, p 1 , 

 is determined experimentally, the principal focal distance of the 

 lens, /, may be determined by the formula, or if any two of the fac- 

 tors, p, p 1 , and /, are known the third may be reckoned from the 

 formula. 



Formation of an Image by the Eye. As stated above, the re- 

 fractive surfaces of the eye act essentially like a convex lens. As a 

 matter of fact, these refractive surfaces are more complex than 

 in the case of the biconvex lens. In the latter the rays of light 

 suffer refraction at two points only. Where they enter the lens 

 they pass from a rarer to a denser medium and where they leave the 

 lens they pass from a denser to a rarer medium. At these two 

 points, therefore, they are refracted. In the eye there is a larger 

 series of refractive surfaces. The light is refracted at the anterior 

 surface of the cornea, where it passes from the air into the denser 

 medium of the cornea ; at the anterior surface of the lens, where it 

 again enters a denser medium; and at the posterior surface of the 

 lens, where it enters the less dense vitreous humor. The relative 

 refractive powers of these different media have been determined 

 and are expressed in terms of their refractive indices, that of air 

 being taken as unity.* 



The three points at which the light is refracted are indicated 



* The term index of refraction expresses the constant ratio between the 

 angles of incidence and of refraction, or specifically between the sine of the 



angle of incidence and the sine of the angle of refraction: -: = index of 



smer 



refraction. 



19 



