REFRACTION OF LIGHT. 40? 



631. Virtual Image. If the object be placed 

 nearer the lens than the principal focus, the image will be 

 virtual, magnified and erect. (Fig. 312.) This explains 

 the familiar magnifying effects of convex lenses. Con- 

 struct the image. 



632. Image of Concave Lens. Images formed 

 by a concave lens are virtual, smaller than the object and 

 erect. The construction of the image is shown in Fig. 

 313. 



FIG. 313. 



Note. The power of the convex lens to form real and diminished 

 images of distant objects and magnified images of near objects, is 

 of frequent application in such optical instruments as the micro- 

 scope, telescope, magic lantern, lighthouse lamps, etc. Owing to 

 the identity between heat and luminous rays, a convex lens is also 

 a " burning-glass." 



633. Spherical Aberration. The rays that pass 

 through a spherical lens near its edge are more refracted 

 than those that pass nearer the centre. They, therefore, 

 converge nearer the lens. A spherical lens cannot refract 

 all of the incident rays to the same point. Hence 

 "spherical aberration" and its annoying consequences in 

 the construction and use of optical apparatus. 



