In many experiments, the cards were displayed face up, but hidden behind a small wooden shield. Several ways of obtaining information about the design on the card remain even in the presence of the shield. For instance, the subject may be able sometimes to see the design on the face-up card reflected in the agent’s glasses. Even if the agent isn’t wearing glasses it is possible to see the reflection in his cornea. In 1938, Harold Gulliksen wrote that Rhine did not describe his experimental methods clearly and used inappropriate mathematical procedures which overestimated the significance of his results. Rhine published Extra-Sensory Perception After Sixty Years in 1940 with a number of colleagues, to address the objections raised. In the book, Rhine and his colleagues described three experiments—the Pearce-Pratt experiment, the Pratt-Woodruff experiment and the Ownbey-Zirkle series—which they believed demonstrated ESP. The psychologist C. E. M. Hansel wrote "it is now known that each experiment contained serious flaws that escaped notice in the examination made by the authors of Extra-Sensory Perception After Sixty Years". Rhine's experiments into psychokinesis (PK) were not replicated by other scientists. John Sladek wrote: His research used dice, with subjects 'willing' them to fall a certain way. Not only can dice be drilled, shaved, falsely numbered and manipulated, but even straight dice often show bias in the long run. Casinos for this reason retire dice often, but at Duke, subjects continued to try for the same effect on the same dice over long experimental runs. Not surprisingly, PK appeared at Duke and nowhere else. The science writer Martin Gardner wrote that Rhine repeatedly tried to replicate his work, but produced only failures that he never reported. Gardner criticized Rhine for not disclosing the names of assistants he caught cheating: His paper "Security Versus Deception in Parapsychology" published in his journal (vol. 38, 1974), runs to 23 pages... Rhine selects twelve sample cases of dishonest experimenters that came to his attention from 1940 to 1950, four of whom were caught 'red-handed'. Not a single name is mentioned. What papers did they publish, one wonders? This has suggested to Gardner that Rhine practiced a "secrecy policy". Gardner mentioned inside information that files in Rhine's laboratory contain material suggesting fraud on the part of Hubert Pearce. Pearce was never able to obtain above-chance results when persons other than the experimenter were present during an experiment making it more likely