Page:Handbook of Ophthalmology (3rd edition).djvu/121

Rh a binocular examination, to employ two concave mirrors; these are placed in a frame provided with a handle, and are separated from each other by a distance corresponding to the distance between the two eyes; each mirror revolves about both a vertical and a horizontal axis. It is only necessary, then, to so arrange the mirrors that the two images of the light cast by them fall upon each other, in order to be able to see ophthalmoscopically with both eyes. Cloudiness of the lens or vitreous, and many cases of detachment of the retina, may be observed binocularly in this manner under the most favorable circumstances. This method is not available for objects lying deeper in the eye, nor for examination in the inverted image.

The source of light for the ophthalmoscopic examination may be either a bright oil- or gas-flame, or daylight. The last, as has been shown by Helmholtz, is the best light for all cases where it is desirable to judge of the true color of the fundus, or to detect slight cloudiness in the retina. Moreover, it has the advantage of dazzling the patient less than lamp-light.

Since, in general, daylight is preferred to artificial illumination in cases where it is wished to recognize fine shades of color, it of course suggested itself to make use of this advantage in ophthalmoscopic examination. It is noticed that by daylight the ophthalmoscopic image has a yellow shade, which disappears entirely by artificial illumination. Under normal relations, this effect is seen most distinctly upon the optic nerve.

The more exact recognition of shades of color and differences in illumination possible by daylight, is probably the reason why slight cloudiness, especially in the retinal substance, which cannot be seen by artificial illumination, becomes visible by daylight. Eyes absolutely physiological often furnish proof of this fact.

The illumination by daylight is best arranged by allowing it to enter a dark room through an opening 3 or 4 centimetres in diameter, cut in the window-shutter. The ophthalmoscope may then be directed toward the sky just as is the mirror of the microscope. Direct sunlight ought, of course, not to be used except in eyes absolutely blind.

The use of daylight is particularly applicable to examinations