The author’s interest in the therapeutic use of prism’s was first aroused about twenty years ago by Dr. Gregory Flynn who used them in the treatment of post-operative convergence in overcorrected intermittent divergent squint. He slightly under corrected the deviation with prisms fitted to the patient’s glasses with special clip-on frames. The patient was seen daily by the orthoptist who gave fusional vergence treatment. She gave details of the smallest fusion angle to Dr. Flynn who then saw the patient and, if possible, reduced the strength of the prism correction. The difficulty with this type of treatment, especially for an orthoptist in private practice is that a large collection both of prisms and various clip-ons to fit spectacles of different types and sizes are necessary. As spectacles become more varied in shape this proved an almost insurmountable barrier and when many children object to wearing the clip-ons because of their weight and appearance this type of treatment was largely abandoned.