18 AMES, PROCTOR AND AMES. 



In all simple lenses this characteristic image formation is more or 

 less confused by coma, a one-sided blur, and by chromatic aberrations. 

 The magnitude of these aberrations in the eye has not yet been 

 measured. 



This may all be made a little clearer by a brief description of how 

 Mr. Ames measured the oblique astigmatism in his own eye. A point 

 in space is fixated with one eye, i.e., by focusing on a point of light, the 

 line of vision and the focus of the eye is not allowed to vary. At one 

 side of the line of vision (at an angle of obliquity of five degrees), three 

 narrow tangential lines (in this case vertical) of yellow light were 

 moved back and forth. It was found that when these lines were at a 

 certain distance they could be distinguished as separate. At any other 

 distance they could not be distinguished. The distance at which they 

 could be distinguished was such as to cause that part of the image 

 designated as aa, Figure 22, to fall on the retina which made the tan- 

 gential lines appear most sharp. This point was found to be nearer 

 than the fixation point. Similar points were found at varying degrees 

 of obliquity from the axis where the three narrow lines appeared most 

 sharp. In this way a surface in space was determined where yellow 

 tangential lines were most visible. The shape of this surface which is 

 called the primary astigmatic object field for yellow is shown in plan 

 on Figure 23 by the solid lines extending from the point marked 

 "fixation point" back towards the eye. 



In the same way a surface in space was determined where yellow 

 radial lines were most visible. This is called the secondary astigmatic 

 object field for yellow and is shown in plan by the dotted lines extended 

 in shape of a ram's horns from the point marked "fixation point" 

 outward. Figure 23. 



Corresponding fields for red, blue and green monochromatic light 

 were found and are shown in Figure 23. 



If this fixation point is changed although these primary and second- 

 ary astigmatic object fields keep their general shape they shift forward 

 and backward. 



Heinrich 7 made a similar experiment with a black thread. He 

 moved a black vertical thread (as he worked in a horizontal plane this 

 would be a tangential line) which was placed to one side of the line of 

 vision, back and forward until it appeared most like the thread on 

 which the eye was fixiated. He found a field similar in shape and 

 position to that which the writer found for the primary astigmatic 

 field for yellow. 



7 British Journal of Psych., Vol. 3, p. 66. 



