18 VEGETABLE HISTOLOGY. 



The rajs around the axis of the lens will meet in a focus C, 

 while those near the edge will meet in C', and a screen placed 

 at C will not receive a mere point 

 of light, as would be the case if the 

 lens were perfect, but a small cir- 

 cle of light. The result of this is 

 that the image of any object is not 

 sharply defined, but is somewhat 

 blurred in such a manner that 

 when the center of the image is v Fig 17 



sharp the edge is indistinct, and ; - 

 when the edges are sharp the center is indistinct. This defect 

 is due to the spherical nature of the lens, hence its name. As 

 the edge rays are most effective in causing this aberration, the 

 latter can be greatly corrected by cutting out the edge rays by 

 means of a diaphragm or perforated disc placed in front of the 

 lens. This is done in objectives and eye pieces of compound 

 microscopes. 



Mathematical calculation has shown that spherical aberra- 

 tion is greatly reduced when the radii of curvature of a lens bear 

 a certain ratio to each other, namely, 6 :1, the face with longer 

 radius being turned towards flie object. Aberration is also 

 corrected in part by combining several lenses of suitable curva- 

 tures into a system, the lens next the object being plano-convex, 

 with the plane face towards the object. (Absolute correction 

 for spherical aberration is impossible.) 



CAUSE OF CHROMATIC ABERRATION. We have seen that rays 

 of different colors have different indices of refraction, i. e., 

 unequal refrangibilities, so that if white light is passed through 

 a prism the constituent colors are separated by it into a spec- 

 trum. A lens acts like a prism in this respect ; in fact, it may 

 roughly be considered as two prisms with their bases together. 

 There is a different focus for each of the seven different colors 

 composing white light; violet being most refracted, is focused 

 nearest the lens, while red, being least refracted, is focused 

 farthest from the lens (Fig. 18). The red rays will meet at 

 R, the violet ones at V, and the other colors at points inter- 

 mediate, in the order, orange, yellow, green, blue, indigo. The 

 result of this defect is that the image of an object is bordered 

 by a color fringe instead of being perfectly colorless, as it 

 should be. Chromatic aberration is more perceptible in pro- 

 portion as the lenses are more convex, i. e., as the magnifying 

 power increases. It is corrected by combining lenses made 

 from crown and flint glass. The refractive indices of these are 

 very nearly the same, being 1.751 for flint and 1.53 for crown, 

 but the power to separate the colors of white light is nearly 

 twice as great for flint as for crown glass. Hence a biconcave 

 or plano-concave flint glass may be so combined with a biconvex 



