16 



SCIOPTICON MANUAL. 



in having them proceed from a point too distant to be 

 shown. The rays of a pencil from a point 100 times 

 further from the lens than is the image, are about paral- 

 lel, and their focus is called the focus of parallel rays, or 

 principal focus. A real pencil is composed of i nnumerable 

 rays, and such pencils from innumerable points in the 

 object meet and cross at the lens on their way to cor- 

 responding points in the image, and wonderful to tell, 

 no one is switched from the track for another, and there 

 are no collisions. An explanation of one answers for 

 countless millions. 



SPHERICAL ABERRATION. 



It is seen (Fig. 6) that the marginal rays d d must be 

 more refracted, or bent, than the more central rays / /, 



Fig. 6. 



in order to meet the axial rays at / , and so it is seen 

 that the margin of the lens G D has a greater refracting 

 angle than the more central portions. But the trouble 

 is, the refracting at the margin is overdone, so that the 

 rays d d meet the axial ray at / 8 instead of at/ r Hence 

 if a ground-glass has been placed at/ t , the marginal rays 

 which have intersected the axis at / 8 will form a circle 

 of dispersion about f t . The diameter of this circle is 

 called the lateral aberration, and the distance between 

 / 8 and/ t is called the longitudinal aberration. As a con- 



