# To obtain a convex achromatic combination with 2 lenses in contact, it is necessary to combine a convex lens of lower dispersive power with a concave lens of higher dispersive power. Is it true? If so, could anyone tell me the reason for this?
Basically yes. The total dispersion is a funtion of both the dispersion of the glass and the focal length of the lense. For two lenses in contact and glass of equal dispersion, the only way to get a zero total dispersion is for the lenses to have focal lengths that cancel. For example a 50 mm positive convex and a -50 mm concave, so the two don't produce an image. (infinite fl)