16 AMES, PROCTOR AND AMES. 



natural effect of depth can only be produced by a lens which has the 

 same depth of field as the eye. 



He does not, however, deal with the effects of chromatic aberration. 

 The depth effect due to it is believed to be much more marked than 

 that due to simple depth of field. 



In simple depth of field the diffusion of edge due to an object being 

 out of focus gives no indication as to whether the object is nearer or 

 farther than the focus point. Chromatic aberration on the other hand 

 causes objects beyond the focus point to be imaged in a characteris- 

 tically different way from those inside. This in turn gives a real basis 

 for monocular depth perception apart from the relative sizes of 

 objects or disappearing or other perspective. As shown by Figure 17a 

 this effect of depth can be obtained in a picture if the objects are de- 

 picted with their characteristic chromatic edges. A marked effect of 

 depth has thus been obtained in pictures painted by Blanche Ames. 



Many paintings by great masters have been looked at to find 

 whether this characteristic edging has been made use of. The treating 

 of edges as stated before is quite common, and in certain paintings, 

 by Millet for instance, warm or reddish edges are found around near 

 objects while objects in the distance are colder and bluer. It is not 

 felt, however, that the evidence is sufficient to conclude that it has 

 been used consistently. 



Chapter III. 

 DEPTH OF FIELD (LATERAL). 



In this chapter we will deal with the characteristics of images of 

 objects which are situated not on the line of vision. The characteristic 

 form of such images is due primarily to an aberration called oblique 

 astigmatism. (This should not be confused with corneal astigmatism.) 



Oblique astigmatism causes the rays of light, from a point source 

 not on the axis, to focus into a ray bundle of complex form. As ex- 

 plained above the ray bundle from a point source on the axis focuses 

 in the form of a cone to a point where the rays cross to spread out into 

 another cone. The ray bundle from a point source not on the axis 

 forms a more complicated figure. In its pure form in a simple un- 

 corrected lens it focuses first to a line, see aa, Figure 22, which lies in a 

 position tangential to a circle about the axis of the lens. It then 

 crosses and narrows in its long dimension and lengthens in its short 



