100 



PRACTICAL MICROSCOPY. 



It will be seen that a double convex lens has a principal 

 focus on either side of it, and therefore the light may be 

 parallelised on either side, but if the source of illumination 

 be placed further away than the principal focus, the rays 



FIG. 90, 



will be no longer parallel, but centred in a point at some 

 distance from the opposite side of the lens, as shown in 

 Fig. 91. 



FIG, 91. 



These points (/ and L) are called conjugate foci, and do 

 not lie in any fixed plane, but are dependent the one upon 

 the other; it is this movement of the conjugate foci which 

 yields a long working distance from the objective when the 

 body of the microscope is shortened, and requires the 

 object-glass to be approached nearer to the object when 

 the draw-tube is used. 



Diverging rays can be produced by placing the illumi- 

 nating point between the principal focus and the lens, and 



