Methods for Improving Spherical Correction. BijProf.Ahhe. 815 



for correcting the residual defects spoken of aiypearecl to be im- 

 possible, unless the connection between the correction of spherical 

 and chromatic aberration could be removed. 



On this supposition, which I afterwards found to be premature, 

 I began to investigate the method of independent correction, which 

 had never been considered before, I discussed the conditions for 

 correcting the spherical aberrations of the convex lenses, or at 

 least a part of these aberrations, by such other concave lenses as 

 produce chromatic correction, and for this pm-pose searched for 

 the appropriate means in systems not restricted to the low aperture 

 of telescopic objectives. The general method could be easily indi- 

 cated, as everything appeared to depend on one essential condition : 

 viz. concave sm-faces which would introduce negative spherical 

 aberration, by diftereuce of the refractive index of consecutive 

 media, as in ordinary binary lenses, but would either exclude 

 chromatic aberration of perceptible amount, or admit chromatic 

 aberration of opposite character. The accessory requisites annexed 

 to this would be somewhat different in the two cases, but would 

 be accomplished without difficulty. The principal condition, how- 

 ever, indicated above, would require optical media essentially 

 different from those hitherto applied. It would be necessary 

 to dispose of two media which are to some extent unequal in 

 refractive power, but either approximately equal in dispersion, 

 or varying in opposite directions. Now, the various kinds of 

 crown and flint glass differ widely in refractive index and in 

 dispersive power, but the dispersion always varies with the index, 

 the greater dispersion being associated with the higher index, and 

 vice versa, with very slight deviations. An independent correction 

 of spherical aberration would require at least two kinds of glass, 

 having optical relations different to those now in use — either low 

 refractive index combined with high dispersive power, or high 

 refraction with low dispersion. There is no reason for supposing 

 the production of such a material an impossibility ; but the makers 

 of optical glass not having even thought of the problem at the 

 time, any improvement of optical systems in the way proposed 

 appeared to be postponed indefinitely. 



Notwithstanding these difficulties, the principle thus indicated 

 of independent correction of both aberrations has been put to a 

 decisive practical trial. In order to obtain a clear view of the 

 direction in which further improvement in the dioptric performance 

 of objectives ought to be made, Mr, C. Zeiss, on my suggestion, under- 

 took a very interesting and important experiment, not hitherto 

 pubhcly recorded, by constructing some systems with jiuid lenses, 

 based on the principle of independent correction. For this purpose 

 we availed om-selves of the low refractive indices of certain of the 

 highly dispersive fluids which are to be found among ethereal oils 



