VISION AND THE TECHNIQUE OF ART. 17 



one until it becomes a line again, see bb, Figure 22, which is perpendicu- 

 lar to the first line. 



The image of every point source not on the axis has this peculiar 

 form. The farther the source from the axis the greater the separation 

 between the two parts of the image which have the form of lines. If a 

 sensitive plate or ground glass screen is placed behind the lens the 

 form of the image that is apparent depends upon the position of the 

 screen. If it is placed far back, i.e., to the left of bb, Figure 22, the 

 image will be in the form of a radial oval, i.e., radial to a circle about 

 the axis of vision, in the horizontal plane this would be horizontal; 

 if at bb, in the form of a radial line; if half way between aa and bb, 

 the form of a circle; if at aa, the form of a tangential line, i.e., tangen- 

 tial a circle about to the axis of vision, in the horizontal plan this 

 would be vertical; if still nearer the lens, in the form of a tangential 

 oval. If the screen is held stationary relative to the lens, a similar 



Figure 22. Diagram showing characteristic shape of the image bundle of 

 a point source not on the axis. 



change in form of image can be noted by moving the point source from 

 a distance to a position near the lens, always keeping it at the same 

 angular obliquity. 



If, instead of a point, a line source is used, a similar imaging takes 

 place. Every point on the line source is stretched tangentially or 

 radially, depending upon the position of the line source. It can be 

 seen that if the stretching of the various parts of the line source is along 

 the length of the line source itself, the image of the line will appear 

 perfectly sharp and slightly elongated. That is, if a line source is tan- 

 gential to the axis, its image will be sharp when the source is so posi- 

 tioned that the part of the image that forms a tangential line, i.e., aa, 

 Figure 22, falls on the screen. If it is in a position radial to the axis, 

 its image will be sharp when that part of its image that forms a radial 

 line falls on the screen. 



