Is it possible to "dope" glass to change it's refractive index? For many purposes, like in the making of achromatic lens combinations, we need transparent media of different refractive indices. Let's assume I want a transparent medium of refractive index of 1.4, how would I go about getting such a material? Can I "dope" it by dissolve some impurities into molten glass mixture and then solidify it later? How would I choose my impurity to add? How do I calculate how much to add?
Glass doesn't crystallize at all, that's the point. Consequently, unlike crystalline materials, it doesn't have fixed composition. In a way, it can be said that glass consists of impurities. And yes, of course, you may change its composition this way or that, so as to tune the refractive index to your purposes.

Further reading: crown glass, flint glass.