Lengthy research was undertaken to determine whether an unknown painting, attributed to the Swiss painter Ferdinand Hodler was an exact replica done by the artist or his students, of one of his works (a common practice of his). Scientific examinations using x-ray analysis, ultraviolet light, infrared reflectography, technical, and chemical testing did not provide enough information to establish with certainty the authenticity of the work; however historical documents proved that the painting originated in Hodler's workshop. As a last resort the confrontation of the original and its copy in transmitted light provided the needed proof. The same method was used to get a better view of an underdrawing barely visible to the naked eye under an 1887 painting (<i>Les Tournesols</i>) by Vincent van Gogh: photography by transmitted light using an electronic flash completed the information obtained from other examinations. -- AATA