THB PRESENT CONDITION OF ENGLISH OBJECT-GLASSES. 265 



Example. — In the finest objectives now obtainable I find 

 when the spherical aberration is corrected there is a strong 

 secondary spectrum, chiefly consisting of a mixture of the red 

 and yellow rays. But when this is corrected, by using all 

 possible precautions, so as to render the achromatism almost 

 absolutely perfect, then the aberration reappears. In point 

 of optical fact the focal point where the spherical aberration 

 vanishes does not correspond with, or is not identical with, 

 the focal point where the coloured rays are blended into 

 white light. But as all the makers more readily detect im- 

 perfect achromatism than residuary aberration, the latter is 

 sacrificed to the former. All the glasses with which I am 

 acquainted err in this respect. 



This important point may appear soinewliat more intelli- 

 gible by referring to figs. 1 and 2. 



In fig. 1 all the rays of every colour ai-e supposed to be 

 idtimately united at the point u, the rays being supposed to 

 be forming a miniature point at B ; aberration, spherical and 

 chromatic, are both destroyed. 



Ln tig, Ji, on the contrary, the red rays are cutting Liiu 



