how can we measure the diameter of the field of view under a microscope using metal wires of 0.5mm in diameter?

Pack the wires side-by-side so they are parallel and touching.

View them through the microscope - you will see a set of parallel lines (each line is a wire).

Count how many lines you can see, N.

Diameter of the field of view = N x 0.5 mm

First, measure the diameter of the pencil. Then wrap the wire around the pencil for round 10 turns. Measure the size of the series of coils. The diameter of the wire is proportional to the length of the turns and the diameter of the pencil. Higher results will probably be received with a cylinder when you consider that most pencils are hexagonal in cross part. Nevertheless, increasing the quantity of coils will support the accuracy of the reply.