INTRODUCTION TO PHYSIOLOGICAL OPTICS 407 



3. Obtain a luminous point as follows. In- 

 sert in front of the condenser the diaphragm with 

 2 mm. aperture. Place the glass slide over the 

 window of the box. Pull out the draw-tube to 

 make the pencil of rays convergent. Throw this 

 convergent pencil into the box. Determine its 

 focus by rinding the place at which a clear image 

 of the aperture of the diaphragm is formed upon 

 a screen. This focus will serve as a luminous 

 point. 



After converging to the focus, the rays will 

 diverge again. Place the mirror 2.5 cm. from the 

 luminous point. The luminous point will then 

 lie at the principal focus of the mirror. Turn 

 the mirror at a small angle with the axis of the 

 pencil. 



The reflected rays will be parallel. 



Conjugate Foci. 1. Place the mirror at a dis- 

 tance from the luminous point greater than the 

 radius of curvature of the mirror. 



The diverging incident rays will be reflected 

 from the spherical surface to a point between the 

 luminous point and the mirror. At this point a 

 real image of the luminous point will be seen. 



The point from which the rays diverge, and 

 the point to which they converge by reflection 

 from the mirror are termed conjugate foci. 



2. Draw back the lantern and thus increase 



