INTRODUCTION TO PHYSIOLOGICAL OPTICS 425 



as close as possible to the convex lens t Let the 

 curvature of the cylinder be in the vertical 

 meridian. 



The circle will give place to a vertical line. 



Move the screen about 4 cm. nearer the lenses. 



The image of the circle will now be a horizon- 

 tal line. 



Place the screen half way between the nearer 

 and the farther focal lines. 



The image will be circular. At other points 

 in the focal interval or space separating the two 

 focal lines the image will be an ellipse. 



2. Hang the block containing the cylindrical 

 lens on the end of the draw-tube of the lantern. 

 Leave the convex lens in its former position. 

 Fill the box with smoke. Let the curvature of 

 the cylindrical lens be in the vertical meridian. 

 Observe the pencil of rays. 



The pencil will be drawn out to a vertical line 

 at the farther focus. Seen from above the cross- 

 section of this line will be a bright spot. At the 

 nearer focus the pencil will be flattened to a 

 horizontal line. 



Kotate the cylinder through 90. The curva- 

 ture will now be in the horizontal meridian. 

 Watch the pencil as the lens turns. 



As the cylinder revolves the contour of the 

 pencil will change. When the curvature is 



