The common terms are endoscopy or sigmoidoscopy or endoscope.
Sigmoidoscope
Endoscope
Colonoscopy is an endoscopic medical procedure that uses a long, flexible, lighted tubular instrument called a colonoscope to view the rectum and the entire inner lining of the colon (large intestine).
instrument used to view the eye
arthroscope
telescope
Binocular
A microscope.
To view things from far away is called a binocular
Retinascope
our naked eyes...
A telescope.
periscope
A microscope