OPAQUE OBJECTS. 



197 



the lens, D E ; these rays, as also those shown by fig. Ill, will 

 be brought into a focus, F, when the lens is placed as there 

 represented; but if the flat side occupy the same position, 

 then, according to fig. 112, it will be in the worst possible 

 condition, as its spherical aberration will be the greatest. 

 Fig. 125 represents this arrangement, the same letters being 

 used as in the preceding figure. Here it will be seen that the 



Fig. 125. 



rays nearest the margin of the lens, D E, will be 'brought to 

 a focus upon the object, a, but those nearer the centre will 

 pass on to F, so that a will be only illuminated to about one- 

 half the extent of a in the preceding figure. When, however, 

 it is required to procure either parallel or diverging rays, 

 so as to illuminate a large surface, such as the mirror or 

 side reflector, the lens must then be placed very near the 

 lamp with its flat side towards the light, as shown in fig. 126, 

 and, according to the distance of the one from the other, 

 so will the rays either be convergent, divergent, or even 

 parallel. Should it be required to illuminate a large portion 

 of an object for dissection, or for other purposes, then we may 

 have recourse to the following plan: the large condenser 

 must be placed near to the lamp, so that diverging rays given 

 off from it may be so converged as to fill the entire circle of 



