When you are hatching chicken eggs, it's difficult, if not impossible to see how things are going inside the egg from the outside. Candling is the process of looking inside the egg without disturbing the developingembryo. By using a bright light it is possible to look into the egg and make sure it is developing properly.
Egg Candling Device
A basic candling device can be made with any bright light shining through a hole of about one inch in diameter. In a dark room, the egg is placed over the hole to block all the light from escaping. The result of this is that the light penetrates the egg shell and lights up the egg from the inside. Brighter lights work better, but bear in mind that powerful light bulbs give off quite a bit of heat and may even set fire to cardboard or other combustibles. Compact Fluorescent Bulbs (CFL) should NOT be used for this because of the risk of breaking and releasing mercury. Candling will give better results if the room is darkened.