Hastings — Optical Errors of the Human Eye. 403 



I. Utility of Error of Collimation. 



Suppose both eyes directed to a distant surface divided ver- 

 tically into two portions of which the left side is red and the 

 right blue, the point of fixation for each eye being on the 

 line of demarcation. In this case the image of the red surface 

 would be represented by the line r r' in each eye and that of 

 the blue surface would be represented by the lines o b\ If 

 the eyes were sharply focussed for red light, that is, if the 

 retinas corresponded with the surfaces r r\ the observer would 

 have a . sharply defined image of the boundary in red and an 

 ill-defined image in blue as the resultant effect of the diffusion 

 circles due to the chromatic aberration of the eye. The diame- 

 ter of these diffusion circles will bear the same ratio to the 

 diameter of the interior pupil as the distance h r does to 7rJ>, 

 while the centers of these diffusion circles corresponding to 

 points at the boundary between the colored fields will lie on a 

 line on the retina which is represented in the diagram by the 

 projection of this line, namely, by the point where the line irjb 

 extended intersects the retina. It is obvious from inspection 

 that only from the line 7rJ? 1 towards the nasal side in the right 

 eye, and towards the temporal side in the left eye, do we have 

 a full illumination of the retina equal to that of the blue field 

 itself. We see, therefore, that in the case supposed there is a 

 narrow region, relatively dark, bordering the sharply defined 

 edge of the red image in the right eye, while in the left eye 

 there is a corresponding region where the contrast is reduced 

 by a commingling of the two colored lights. 



It is easy to see that if the eye is adjusted for distinct vision 

 of the blue surface, that is, if the retina corresponds to h V of 

 the diagram, exactly the same conditions as regards sharpness 

 of distinction hold as before. If the observer's attention were 

 directed to this particular feature of the object, that is, to the 

 dividing line in the field, he would inevitably accommodate 

 either for the red or for the blue, and, ignoring the confused 

 sensation of the left retina, recognize a sharpness of division 

 which would be lacking if the right eye were symmetrical in 

 construction. If the red and blue fields were interchanged 

 the left eye would become the discriminating one. 



Before we can attach great importance to this conclusion we 

 must see if the relations quantitatively considered are such as to 

 support it. To do this we must calculate the angular width 

 of the darkened strip and of the radius of the diffusion circles; 

 the first giving us a notion as to its conspicuousness as an 

 interruption in the field of vision, and the second a notion as 

 to its intensity. We read at once from the diagram that the 

 true width of the region is — 



