Probably an electron microscope.
A microscope.
Microscopes coupled with techniques used to separate them from inside the cell.
Protein is used inside cells to protect the cell and provide nutrients.
A light microscope is typically used to see the structures inside a cell, as it provides enough magnification to observe organelles like the nucleus and mitochondria. Electron microscopes can also be used for higher resolution imaging of cellular structures.
I believe the answer is the "plasma membrane"!
The most common way to look inside a cell is to use a microscope. There are several different kinds of microscopes (fluorescent microscopes, electron microscopes, light microscopes) as well as several different kinds of contrast generating techniques (phase contrast, DIC) which help to see different parts of the a cell more clearly. Often times a dye is also used to help visualize the inside of a cell.
Inside of a cell it is the cell membrane which lets substances pass into the cell, and out. The cell wall is used to keep the shape of the cell the same.
Transmission Electron Microscope is used to take an image of a cell and split it and get information from the inside of the cell
The cell membrane (plasma membrane) is inside the cell wall. You can sometimes see the cell membrane if you look carefully but its right under the cell wall
X-ray is used to see inside concrete and is often used to test for cracks in high stress applications.
Ultrasound
Cell phones are used to see e-mails and to keep in track