A new method of varying prism power is used to examine the vergence break performance of a limited group of subjects. All sources of error or delay other than the opinion of the subject are effectively eliminated by the use of a programmed digital computer. Repetitions are made at a rate previously impossible. The sequence of responses given by each subject seems to reveal a characteristic of his visual system. Selected subjects were retested after several weeks and showed a response pattern broadly similar to that observed initially.