590 PHYSIOLOGY 



A lens with a focal distance of 1 metre is taken as unit strength 

 Such a lens is said to have a strength of one dioptre ; a lens of two 

 dioptres would have a focal distance of J metre ; of four dioptres one 

 of J- metre, and so on. 



DIOPTRIC MECHANISMS OF THE EYEBALL 

 The eye is not directly comparable to a camera in its optical 

 arrangements, since the image is formed, not in air, but in the fluids 

 of the eye itself. Moreover the conditions are complicated by the 

 facts that it is 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 



A f c 



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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. 259 (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 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 nodal 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 a 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 (PI) 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 (qrar) meets 

 this. (In this case the angle^formed by the rays c to c 2 with CQ must be very 

 small.) The propositions 1 and 2, of course, may be reversed ; the divergent rays 

 proceeding from p towards ab pass into the first medium parallel to each other, 



