Page:Optics.djvu/95



then making $(∆−r)·(∆‵−rv)d∆‵−r(∆‵−r)dv⁄√∆‵^{2}−2r(∆‵−r)v$, $=m√ ∆^{2}−2r(∆−r)v¯¯¯¯¯¯¯¯¯¯ ·d∆‵−m(∆‵−r)·r(∆−r)dv⁄√∆^{2}−2r(∆−r)v$,

{{c|that is, $v=0$}

When the incident rays are parallel, or $∆‵=∆′$, this reduces to

98. In general, the aberration is positive or negative, that is, $(∆−r)·∆′d∆‵−r(∆′−r)dv⁄∆′$ is greater or less than the ultimate value, according as $=m∆d∆‵−m(∆′−r)·(∆−r)rdv⁄∆$ is positive or negative.