Existing comment:
Burning glass/magnifier:
18th century
Owned by Benjamin Franklin
In June 1752, Franklin asked his friend William Strahan to purchase for him "a Concave Mirror or Burning-Glass," which he used to conduct experiments on the heat absorbency of light and dark material. Encouraging his young friend Polly Stevenson in her education, he wrote, "Try to fire Paper with a burning Glass. If it is White, you will not easily burn it; but if you bring the Focus to a black Spot... the Paper will immediately be on fire under the Letters." The burning glass was also used as a simple magnifier. |