Hold an egg up to a light and you can see what is going on inside. You can check infertile eggs for blood spots, and the shell for fine cracks. Fertile eggs can be inspected for signs of life and embryo development. Years ago, eggs were held up to a flame, today it’s more likely to be a flashlight or lightbulb, but this method is still called candling.
This print appeared in The Lady’s Friend, published in 1865.