INTRODUCTION TO PHYSIOLOGICAL OPTICS 429 



diverging from the illuminated aperture pass 

 through the refracting cylinder. 



The curvature of the caustic surfaces will be 

 more noticeable than in Experiment 2. 



Dispersion Circles. 1. Let tl.e parallel rays 

 pass through the double convex lens. Place a 

 screen at the principal focus. A clear image of 

 the circular aperture in the diaphragm will be 

 seen. Move the screen away from and then 

 towards the lens. 



When the screen is either nearer or farther 

 from the lens than the principal focus, the image 

 will be larger and less distinct. The screen will 

 cut the pencil in the one case before it has con- 

 verged to the focus, and in the other case after 

 it has passed the focal point and is diverging. 

 Under such circumstances the image of a point 

 becomes a circle, termed a dispersion circle or 

 circle of confusion. 



2. Substitute the diaphragm with L-shaped 

 aperture for that with circular aperture. Place 

 the screen a little nearer or farther than the 

 focal point. 



The image will be a broad blurred line with 

 convex ends. The pencils proceeding from each 

 luminous point in the line will fall upon the 

 screen in dispersion circles. The broad line is 

 caused by the overlapping of the dispersion cir- 



