Why are blood smears made with a feathered edge?

When a patient needs certain blood work done for testing of potential conditions, one way of testing the blood is by doing a blood smear. A blood smear is done by doing a finger prick, extracting a drop of blood, and placing the drop of blood on a glass laboratory microscope plate. Once the drop of blood is on the glass plate, a separate glass plate is used to spread the drop of blood out. It is done by "smearing" the blood across the bottom plate. The desired and only lab-accepted "smear" results in a feathering of the blood, or a increasingly thinning of the amount of blood across the plate, in turn creating a feathered appearance of the blood on the plate. It helps the lab chemists look at the right amount of blood for determining the results of the blood test.