10 AMES, PROCTOR AND AMES. 



at the purple railroad switch lamps used, for example, by the Boston 

 and Albany Railroad. At close range these lamps appear purple, 

 but as one moves away the light appears to have a red center sur- 

 rounded by a blue disk or halo; the farther off one goes the larger 

 the blue halo appears. 



SPHERICAL ABERRATION. 



Another factor that has a bearing on the nature of the retinal image 

 of an object on which the eye is focused is the spherical aberration 

 mentioned above. In a lens free from spherical aberration the rays 

 that come through different zones of the lens, for example those that 

 come through the lens near its center and near its edge, all focus down 

 to a point. This is shown in Figure 5 where rays near the outside of 

 the cone and those near its center all go through one point at the apex 

 of the cone. In a lens which has spherical aberration this is not true. 

 The rays from different zones of the lens do not pass through the same 

 point. This is shown irt Figure 3, where it will be seen that ray A, for 

 instance, crosses the axis far to the left of ray B. Due to this fact the 

 image of a point source formed in the eye will be larger and with softer 

 edges than that formed by a corrected lens. 



IRREGULAR ASTIGMATISM. 



There is still another factor that has a bearing on the nature of 

 retinal images of an object on which the eye is focused. That is 

 irregular astigmatism. This term covers all irregularities in the shape 

 and distribution of light in the image due to such things as opaque 

 substances or irregularities of densities in the lens system. A most 

 marked example of this factor is the star shape appearance, known to 

 everyone, of a small source of light. If the eye were not subject to 

 irregular astigmatism of some sort the image of a small source, though 

 it might be affected by chromatic and spherical aberration, would be 

 circular. Such retinal images, however, are always star shaped. 



SUMMARY. 



The three aforementioned factors, chromatic aberration, spherical 

 aberration, and irregular astigmatism, cause the retinal image of an 

 object upon which the eye is focused to have a characteristic appear- 

 ance both as to the amount of detail which is visible and in the appear- 

 ance of all edges. 



