PROPERTIES OF LENSES. 8 



As the image of an object is only a collection of 

 the foci of the different points of an object, these 

 images will be real or virtual as the image is situated 

 without or within the principal focus. 



A real magnified and inverted image of an object 

 is seen by the eye if the object is situated outside the 

 principal focus of a lens, and at less than twice the 

 focal distance, because the angle of intersection of 

 the rays from opposite sides of the object, i.e. the 

 visual angle, is increased. 



A virtual erect and magnified image is seen by the 

 eye if the object is situated within the principal focus 

 of the lens, because the crystaline lens of the ob- 

 server C Fig. 6 has the power of bringing the rays 

 to a focus and forming a real image on the retina R, 

 on this principal the magnified image in the simple 

 microscope depends. 



Concave lenses can only form virtual images no 

 matter what the distance of the object. (Fig. 7). 



FIG. 7. 



Up to the present the passage of light through 

 lenses has been considered in its simplest form, it 

 remains now to investigate the imperfections in the 



