Eggs are 'candled' by placing them
up against a light source so you can see inside. It should be done
in a darkened room and requires no special equipment. A flashlight
can be turned into a 'candler' simply by cupping your hand around
the beam so your thumb meets your fingers and an egg-shaped
opening somewhat smaller than the egg is made. Place the egg on
the circle made by your thumb and fore-finger so the light from
the flashlight shines through the egg with you looking at the
other side. It will not be possible to see much detail, but you
will be able to determine if the egg is developing normally. . . .
best done in the dark
The eggs should be candled at 7 days' incubation, then again at 14 and 21 days. By four days, the heart will be partially formed and if you are patient you will be able to see it beating. . . a red spider with a blob in the middle. . the heart. At seven days, the eyes are visible as relatively large black dots, and the embryo will be in almost constant movement especially when you shine a light through the shell . At 14 days you should be able to make out a rough outline of the embryo and by 21 days the entire egg will be nearly blacked out. After this time you will gain more information by looking at the two ends of the egg, where you should be able to see occasional movement. Any eggs found to be dead or infertile should be removed from the incubator. Clear (infertile) eggs are fairly easy to spot at 7 days and should be discarded at that time.
On duck eggs if unsure sniff as a dead egg starts to smell pretty quickly ! Do not leave 'dead' eggs in the incubator too long, though. If they are allowed to remain in the incubator, they will rot and have even been known to explode with unbelievable results when you attempt to clean up. The gas will also kill the eggs nearby.
If using for geese eggs then buy the halogen candler as the chaper one does not go through their shell