CH. LVIII.] 



FORMATION OF IMAGES 



823 



drawn from the borders of any object to the nodal point; if the 

 lines are prolonged backwards they include an equal angle. It has 

 been shown by Helmholtz that the smallest angular distance between 

 two points which can be appreciated as two distinct points = 50 

 seconds, the size of the retinal image being 3'65 /x ; this is a little 

 more than the diameter of a cone at the fovea centralis which = 3 /x, 

 the distance between the centres of two adjacent cones being = 4 JUL. 

 If the two points are so close together that they subtend a visual 

 angle less than 50 seconds, both images will fall upon one cone, and 

 the two points will therefore appear as one. 



Any object, for example, the arrow A B (fig. 523), may be con- 

 sidered as a series of points from each of which a pencil of light 

 diverges to the eye. Take, for instance, the rays diverging from the 

 tip of the arrow A ; C C represents the curvature of the schematic 

 or reduced eye ; the ray which passes through the centre of the circle 



Fio. 523. Diagram of the course of the rays of light, to show how an Image 'is formed upon the retina. 

 The surface C C should be supposed to represent the ideal curvature. 



of which C C is part is not refracted; this point is represented as 

 an asterisk in fig. 523 ; it is near the posterior surface of the crystal- 

 line lens ; the ray A C, which is parallel to the optic axis 0', is 

 refracted through the principal posterior focus P, and cuts the first 

 ray at tha point A' on the retina. All the other rays from A meet 

 at the same point. Similarly the other end of the arrow B is focussed 

 at B', and rays from all other points have corresponding focusses. 



It will thus be seen that an inverted image of external objects is 

 formed on the retina. The retina is a curved screen, but the images 

 fall only on a small area of the retina under normal circumstances ; 

 hence, for practical purposes, this small area may be regarded as flat. 



The question then arises, Why is it that objects do not appear to 

 us to be upside down ? This cannot be satisfactorily answered without 

 entering into matters which require a previous psychological train- 

 ing. Suffice it to say here that the localisation of objects in space 

 depends not only on the retina, but also on tactile and general 



