The test with Snellens or other colored letters is also a good one. A word with alternate red and bluish green letters is painted on glass and placed in the window, and the patient is asked to read the letters. If a bluish green glass is held in front of the good eye, he will see only the green letters unless he can see with the other eye, for all but the red rays in the red letters are cut off in the transparency in the window, and the green glass cuts the red off, leaving those letters a perfect blank to the well eye. Kugel places various colored glasses before each eye and then places an opaque glass in front of the sound eye, and a transparent glass of the same color before the other one, and if the patient sees the object, he is simulating blindness. Herring has the patient look through a tube large enough to cover both eyes, and then suspends a small ball in front of the tube and drops small objects near this ball, and if the patient can tell whether the balls are dropped in front of or behind the suspended ball, he must see with both eyes. Lawrence recommends the stereoscope for detecting binocular vision, and places in a covered stereoscope a picture each side of which is different, and yet such as to make a single picture when both sides are seen. A clock dial, for example, with figures in one side only or figures with complemental colors, such that with both eyes the object would appear differently colored from what it would when seen with either eye separately. The distance apart of two objects held up in front of both eyes can be readily told by the patient if he sees with both eyes, no matter how the objects are held with relation to each other. But if there is vision with one eye only, the patient can tell the distant apart with accuracy only when the objects are both held at the same distance from the eye, but not when one is held considerably in front of the other.