Art conservation changed the previously perceived art history of Johannes Vermeer’s 1657 painting “Girl Reading a Letter at an Open Window.” In 1979, art historians took an X-ray of the Vermeer painting, and a framed painting of a nude Cupid appeared in the background. After the 1979 scholars rejected suspicion of tampering, modern historians reevaluated the X-rays in 2017, and several evaluation tests were run. The tests resulted in the criminal discovery that the nude Cupid was covered forty years after Vermeer's death. After the painting of Cupid was revealed, the iconography and symbolism of the prominent factors in the painting, the girl and the letter, changed from reflecting unusual independence to domestic femininity. With the Cupid found to be criminally covered, a microscopic cross-section was done to evaluate the layers further. The 2021 investigation found that the layer of paint to cover the cupid was painted to match the already-aged Varnish used by Vermeer approximately sixty years before the repainting. The difference in the material caused a yellow stain to blemish Vermeer’s original varnish layer. Since Vermeer intended the varnish to add muted tones to create a melancholy and calming effect, when the conservators ultimately removed the varnish layer, they also removed the impact of the muted tones. The conservation team claims they regret the unfortunate decision they had to make as they recognize that future viewers will never experience the full intensity of the painting. A scientific study was conducted in November 2023 on the viewer's visual opinion of the painting “Girl Reading a Letter at an Open Window.” The conclusion was that viewers preferred the before version when they saw the side-by-side comparison of the painting before and after the conservation. Concluding that art conservation can change the previously perceived and the assumed future art history.