INTRODUCTION TO PHYSIOLOGICAL OPTICS 421 



ture being 5 cm., the principal focus will lie 

 approximately 5 cm. from the optical centre. 

 Draw through the optical centre a line 10 mm. 

 long at right angles to and bisected by the prin- 

 cipal axis. Connect the ends of this line with 

 the principal focus. The angle included will 

 be approximately 10. The vertical line will 

 then represent a double convex lens of 10 aper- 

 ture, assumed to be without thickness in order 

 that the nodal points may coincide with the opti- 

 cal centre. At any distance greater than 5 cm., 

 draw an arrow at right angles to and bisected 

 by the principal axis. The height of the arrow 

 must not exceed the diameter of the lens, so 

 that rays emitted from the ends of the arrow 

 parallel to the principal axis of the lens shall 

 pass through the lens. From the ends of the 

 arrow draw to the lens and thence through 

 the principal focus incident rays parallel to the 

 principal axis. From each end of the arrow 

 draw a line through the optical centre of the 

 lens. 



The intersections of these lines mark the upper 

 and lower limits of the image. Note that the 

 image is real and inverted. If the object be situ- 

 ated at twice the focal distance from the lens, 

 the image will be the size of the object ; if at less 

 than twice the focal distance, the image will be 



