INTRODUCTION TO PHYSIOLOGICAL OPTICS 419 



side of the lens, the conjugate focal distances 

 are equal. 



Move the lantern farther from the lens. 



The conjugate focus will approach the lens. 

 As one conjugate focus recedes the other ap- 

 proaches the lens. 



Virtual image. 1. Place the 2 mm. diaphragm 

 over the window of the optical box. Let the di- 

 verging rays pass through the convex lens placed 

 at a distance from the luminous point less than 

 the principal focal distance. 



After passing through the lens, the diverging 

 rays will continue to diverge though the degree 

 of divergence will be less. Prolonged backwards, 

 they would unite in a virtual image on the same 

 side of the lens as the luminous object. The 

 virtual image is farther from the lens than the 

 object, is never inverted, and is always enlarged. 

 (Compare the construction, directions for which 

 will be given on page 421.) 



2. Look through the convex lens at printed 

 words placed between the lens and its principal 

 focus. 



The image is virtual and enlarged. 



Construction of Image obtained with Convex 

 Lens. The line which joins the centres of curva- 

 ture of a double convex lens is termed the prin- 

 cipal axis or optical axis. In every lens there 



