The eye can tell a physician quite a bit about systemic health. However, ophthalmologists and optometrists do not use an xray. This would damage the eye. The doctor uses a slit lamp, a type of microscope with a light, to look at the cornea, iris, lens and optic nerve. He then uses an indirect ophthalmoscope, another microscope of sorts with a light attached that he wears around his head and looks at the retina to see if there are any lesions or bleeding. Typically diseases like hypertension, high cholesterol and diabetes will show signs in the eye before it's found in the body. This is why eye exams are recommended every year.