14 PRACTICAL HISTOLOGY. 



it (C'). It is evident that if a cone of rays proceeded from 

 a luminous point at F, the divergent pencil would be con- 

 verted into a cylinder of parallel rays. If the luminous 



FIG. 13. Principal focus. (Deschanel., 



point were placed within the principal focus say at C', the 

 more divergent cone would not be rendered parallel by the 

 lens, but would still continue to diverge, though to a smaller 

 degree, after passing through it ; but if the luminous pencil 



FiG.-i4. Principal focus (F) and secondary foci (C' D'). (Hannover. 



proceeded from a point outside the principal focus, say at D', 

 the less divergent rays would be rendered convergent 

 (D D) by the lens, and would come to a focus somewhere 

 on the other side of it. In Fig. 1 5 a cone of rays is seen 

 proceeding from a point S', outside the principal focus F, 

 and brought by a lens to a real focus at S. At S a real 

 image of the point S' would be produced. Conversely, if 

 the rays proceeded from S, they would come to a real focus 

 at S', and if S were placed nearer the lens, but outside the 



