Elements of natural philosophy (Volume 2-3) . ELEMENTS OF OPTICS. 205 tween the lens and the principal focus, it will, after Give virtual focLdeviation, proceed from some other point in front, and thefocus will be virtual. Fo] 1 1 _ - j? 1 f = 0, or/ = oo Fis. Real radiant atprincipal focus. F„ f That is, the vergencyof the deviated rayswill be zero, and aspherical wave pro-ceeding from the prin-cipal focus will be con-verted, by deviation,into a plane wave which can only be concentrated into a point at an infinitedistance. If the rays be received by the lens in a state of con- For virtua
Elements of natural philosophy (Volume 2-3) . ELEMENTS OF OPTICS. 205 tween the lens and the principal focus, it will, after Give virtual focLdeviation, proceed from some other point in front, and thefocus will be virtual. Fo] 1 1 _ - j? 1 f = 0, or/ = oo Fis. Real radiant atprincipal focus. F„ f That is, the vergencyof the deviated rayswill be zero, and aspherical wave pro-ceeding from the prin-cipal focus will be con-verted, by deviation,into a plane wave which can only be concentrated into a point at an infinitedistance. If the rays be received by the lens in a state of con- For virtualvergence to a point behind, that is, if the concavityia(of the wave be turned to the front before deviation, then - or f will be negative, and Equation (40), becomes«/ The equationbecomes; Fh r„ \ \ \ \ I III l I I I I | -f-f- f—I—I And the foci will i i i i i always be real. and the vergencyof the deviated rayswill always be ne-gative. In otherwords, to whateverpoint behind thelens the wave maybe tending to con-centration before de-viation, the deviation will cause it to concentrate in someother point behind. If the rays proceed from a point in front and at the ^J^f^distance of twice the principal focal distance, / becomes 2 f„ ;equal to 2 Fjn
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