132 S^pherical and Chromatic Aberration. By Br.Boyston-Pigoit. 



duced by oil of cassia enclosed between two concave lenses, will 

 have a very different relation to that of the red ray, as compared 

 with the aberrations of sulphuric acid similarly enclosed. 



Indeed, the variations of the chromatic and spherical aberra- 

 tions go, as it were, hand in hand. Spherically considered, their 

 characters are identical, but their qualities depend upon the nature 

 of the light and the media through which it is transmitted.* 



(To he continued^ 



Additional Note. January 14, 1876. 



I have been led to the consideration of the subject in conse- 

 quence of the very imperfect notions and ideas afloat regarding this 

 very fundamental principle in optics. The advanced student of 

 science in general is becoming daily better acquainted with its 

 general laws. It was an unfortunate circumstance that for novices 

 it was found convenient to employ the figment that light in optics 

 might be considered homogeneous for the pm-pose of simphfying 

 optical formulae ; this veritable scholastic sham should have been 

 more carefully guarded and explained. The result upon general 

 readers has been lamentable. Spherical aberration, the grand dif- 

 ficulty of opticians, is thought to belong only to a pure homo- 

 geneous ray. Chromatism is represented as cured by regulating 

 the foci of lenses ; whilst chromatic error is represented as having 

 nothing whatever to do with spherical aberration ; spherical and 

 chromatic aberration being thus made distinct and as it were inde- 

 pendent, is pernicious to optical science, as being utterly false. In 

 the standard optical works chromatic aberration is only treated of 

 centrically : the excentrical is altogether omitted. The ques- 

 tion is one of the most important possible in fundamental optics. 

 [I may further remark to-day, February 12, that spherical aberra- 

 tion has no existence for the central ray, but chromatic aberration 

 displaces the focus of the mean central rays. But the moment a 

 coloured ray passes marginally or excentrically, it that instant 

 obeys the laws of spherical aberration : and has its identical cha- 

 racters. ] 



Dr. Parkinson says, in his preface to ' Optics,' that the work is 

 a new edition of Griffin's ' Optics.' To the latter gentleman, both 

 the present and former paper have been submitted, and from him I 



* Suppose a violet ray to pass through the margin of a lens and also through 

 a small central aperture, then its variation in focus is identical with its spherical 

 aberration ; and if al&o a red ray pass similarly, the resulting variation in focus is 

 also the abcrriitiou due to the marginal curvature, so tliat these variations are 

 identical in character as being spherically produced and spherically calculated. 

 —(Note added Feb. 12.) 



