Describes a nodal slide which is inexpensive to construct, being made entirely of plastic. It is flexible, allowing variation in the number of components, their separation and even of the refractive index at different parts of the system. The precision allowed by the scales marked on the plastic components is quite adequate to allow sensible comparisons between thick lens theory and the alternative treatment in terms of multiple application of thin lens theory when lens systems made up from thin lens combinations are used. This facility is invaluable in the teaching of lens theory. Extra scale precision would be inappropriate in view of the inherent errors in judging the positions of the focal and nodal points. Some results obtained in the form of a comparison between experimental and theoretical thick lens parameters are presented.