Do Newton's experiment in the lab. Replace a section of the blackout with black card and cut a hole in it. Produce a spectrum and try to split the red light further with another prism. Ask students what they think will happen. Then get them to do this experiment in small groups with two prisms; use a ray box to allow just the red beam to go through the slit on to the second prism. Stress the huge importance of the results they have reproduced . Quote from Opticks, Dover edition page 26, Proposition2, Experiment 3, or use the Newton Project website. A particularly lovely description of the experiment is in a letter to the Royal Society reproduced at www.newtonproject.ic.ac.uktextslight-colours_n.html
Urge students to think about how ways of announcing results have changed so much.