Another approach to chromatic aberration reduction is to use two positive lenses separated by one half the sum of their wavelengths. Two equal positive lenses are used in eyepieces like the Ramsden eyepiece for correction. This approach to minimizing chromatic aberration uses two lenses of the same type of glass so that there is just one index of refraction n, and the expression for the combined power of the lens combination depends upon that index of refraction. If you take the derivative of the power expression with respect to n and set it equal to zero, you can solve for the required separation of the lenses which makes that derivative equal to zero. The variation of index of refraction with light wavelength or color is called dispersion, and the zero derivative implies that the dispersion is zero. If the power does not depend on the index of refraction n, you have eliminated chromatic aberration. The caveat is that these expressions presume thin lenses and paraxial rays (close...
Optics for Optometry