DIOPTRIC MECHANISMS OF THE EYEBALL :,:j| 



impossible to neglect the thickness of the refractive media, and that these 

 are many in number. 



If we take the simplest case, where there are only two media separated from one 

 another by a spherical surface of contact, we can easily determine the course taken 

 by any ray in passing from the first to the second medium. 



In Fig. 261 (from Landois) let L be the first (e.g. air) and G the second (e.g. glass). 

 These are separated by the spherical surface ab, with its centre at m. Since all the 



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radii drawn from m to ab are perpendicular to the surface all rays falling in the direction 

 of the radii must pass unrefracted through m. All rays of this sort are called rays 

 or lines of direction ; m, as the point of intersection of all these, is called the ncdal 

 point. The line which connects m with the vertex of the spherical surface, x, and which 

 is prolonged in both directions, is the optic axis, OQ. A plane (EF) in x, perpendicular 

 to OQ, is called the principal plane, and in it x is the principal point. The following 

 facts have been ascertained : (1 ) All rays (a to 5 ), which in the first medium are parallel 

 with each other and with the optic axis, and fall upon ab, are so refracted in the second 

 medium that they are all again united in one point (p^} of the second medium. This 

 is called the second principal focus. A plane in this point, perpendicular to OQ, is 

 called the second focal plane (CD). (2) All rays (c to c 2 ), which in the first medium 

 are parallel to each other, but not parallel to OQ, reunite in a point of the second focal 

 plane (r), where the non-refracted directive ray (c L mr) meets this. (In this case the 

 angle formed by the rays c to c 2 with OQ must be very small. ) The propositions 1 and 

 2, of course, may be reversed ; the divergent rays proceeding from pk towards ab pass 

 into the first medium parallel to each other, and also with the axis OQ (a to a 5 ) ; and 

 the rays proceeding from r pass into the first medium parallel to each other, but not 

 parallel to the axis OQ (as c to c 2 ). (3) All rays, which in the second medium are 

 parallel to each other (b to 6 5 ) and with the axis OQ, reunite in a point in the first medium 

 (p) called the first focal point ; of course, the converse of this is true. A plane in this 

 point perpendicular to OQ is called the first focal plane (AB). The radius of the refractive 

 surface (ma?) is equal to the difference of the distance of both focal points (p and p^) 

 from the principal focus (x) ; thus mx = p^x px. 



In compound systems composed of several refractive media with spherical surfaces 

 of contact, such as the eye, we may proceed from medium to medium with the same 

 methods as those just described. Since, however, such a procedure would be very 

 tedious, the method first proposed by Gauss is usually adopted. Gauss showed that 

 if the several media are ' centred ' i.e. if all have the same optic axis then the refractive 

 indices of such a centred system may be represented by two equally strong refractive 



