130 Spherical and Chromatic Aberration. By Dr. Boyston-Pigott. 



used. Potter's term, longitudinal dispersion, is precise and self- 

 evident. The term chromatic aberration has been so loosely em- 

 ployed as to give rise to sufficient confusion. 



Thus chromatic dispersion is simply applied to denote the 

 various ways in which the colours in a solar spectrum are dispersed 

 over its whole length, which vary in their degree, position, and 

 intensity, according to the nature of the light and prisms employed. 



Again, in the standard optical works the chromatic aberration 

 calculated, is merely the variation of the focal length for the central 

 rays forming what is called the geometrical focus, which, mathe- 

 matically speaking, is used only for an infinitely small axial pencil 

 of rays passing through the exact centre of the lens in question : 

 but the chromatic aberration of a given coloured ray joassing through 

 the periphery or marginal area of the lens is altogether omitted, 

 although implied in the fundamental formulae. 



Further, the popular canon in achromatics, that achromatism is 

 determined by the condition that the dispersions of the two achro- 

 matizing lenses must simply be in proportion to their focal lengths, 

 is a rough formula, only true for the geometrical focal lengths : for 

 it is entirely founded on the fundamental value of the focal lengths 

 of the central rays, and even the thickness of the lenses is entirely 

 neglected in this popular canon, and only two colours can be 

 united for the dispersions of the two sets of rays chosen. 



Opticians have determined for themselves the fallacy of this 

 canon for delicate purposes, and of the two necessary evils chosen 

 the least. In forming a telescope of two glasses, they find minute 

 double stars are shown most distinctly when the secondary spectrum 

 or uncorrected colour is faintly purplish, or claret colour. 



On referring to Brewster's treatise on Optics,* the identical 

 character of chromatic and spherical aberration is well implied. 

 He says, p. 79, 



"In treating of the progress of rays through lenses, it was 

 taken for granted that the light was homogeneous, and that every 

 ray that had the same angle of incidence had also the same angle of 

 refraction, or what is the same thing, that every ray which fell 

 upon the lens had the same index of refraction. The observations 

 in the preceding chapters have proved, however, that this is not 

 true, and that in the case of light falling upon crown glass there 

 are rays with every possible index of refraction from 1 • 5258 to 

 1 • 5466, the index of refraction for the violet rays " 



" The extreme red rays (marginal) will have their focus in r, 

 whilst the extreme marginal violet rays whose index of refrac- 

 tion is 1 • 54G6 will have their focus in v. The distance v r is 

 called the chromatic aberration." And I may remark, there is 



* Lavclncr's 'Cab. Cyclop.,' "Optics," ])y D. Biewster, LL.D., F.K.S., after- 

 wards Sir David Brewster. 



