Cover slip or cover glass?

to protect the smear or specimen from damage.

A cover slip is a small, and extremely thin piece of glass used in the operation of a microscope. The cover slip is placed on top of the specimen which is then placed under the microscope. The...

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to keep a speciman or object in place, and it is clear to see through it

It is used to hold the specimen being observed in place. It keeps the microscopic lens from touching the specimen, and it keeps the specimen from being contaminated.