Is it ensure, insure, or assure?
ophthalmoscope oph·thal·mo·scope (ŏf-thāl'mə-skōp', ŏp-)
An instrument for examining the interior structures of the eye, especially the retina, consisting essentially of a mirror that reflects light into the eye and a central hole through which the eye is examined.
instrument for inspecting the interior of the eye, invented in 1850 by the German physiologist Hermann von Helmholtz. The ophthalmoscope became a model for later forms of endoscopy. The device consists of a strong light that can be directed into the eye by a small mirror or prism. The light reflects off the retina and back through a small hole in the ophthalmoscope, through which the examiner sees a nonstereoscopic magnified image of the structures at the back of the eye, including the optic disk, retina, retinal blood vessels, macula, and choroid. The ophthalmoscope is particularly useful as a screening tool for various ocular diseases, such as diabetic retinopathy.