THE MICROSCOPE. 15 



principal focus, S' would recede from the lens. The points 

 S and S 7 are therefore termed real conjugate foci ; that is, 





FIG. 15. Conjugate foci, both real. (Deschanel.} 



points so correlated that rays diverging from the one con- 

 verge to the other. In Fig. 15 the luminous point is 

 placed in the principal axis of the lens ; if it were placed 

 above or below the axis, the rays would be focalised on 

 the opposite side of the axis (Fig. 17). 



25. Virtual Focus. Let Fig. 1 6 represent a luminous 

 point T placed within the principal focus F of the lens 

 A B, the rays are never 



brought to a focus, but 

 are only rendered less 

 divergent by the lens. 

 The rays after travers- 

 ing the lens proceed 

 just as if they eman- 

 ated from the point V, 

 to which they would 



pass if prolonged backwards, without the intervention of 

 the lens. To an observer a luminous object at T would 

 appear to be at V. V would therefore be a virtual but not 

 a real image of the object T, or, otherwise expressed, T is 

 a real, and V a virtual focus. 



26. Real Image. If a luminous object be placed at 

 twice the focal distance from a convex lens, a real image of 

 the same size is produced at the same focal distance ; but 

 if the object (a b, Fig. 17) be placed at less than twice the 



FIG. 16. Real and virtual foci. (Lommel.) 



