You should sprinkle it throughout. Otherwise, the reader will be uninterested if all of the interesting details are in one part of the story and stop reading it.
It symbolizes how one who believes in certain aspects, can look into circumstances, hoping for best outcomes regardless the toughness of the situation.
Same as its to microscope which provided light to somebody who has a sight problem but believes that after putting on the microscope he or she can see through.
The microscope therefore symbolized the power of believing.
