As previously stated, it is practically impossible to accurately diagnose a case of muscular imbalance with trial-case prisms. For this reason the phorometer forms an important part of the equipment for muscle testing in the Ski-optometer, having proven both rapid and accurate. It consists of two five-degree prisms with bases opposite, each reflecting an object toward the apex or thin edge. The patient whose attention is directed to the usual muscle-testing spot of light, will see _two_ spots. Aside from the instrument itself, and in further explanation of the phorometer’s principle and construction, when two five-degree prisms are placed together so that their bases are directly opposite, they naturally neutralize; when their bases are together, their strength is doubled. Thus while the prisms of the phorometer are rotating, they give prism values from plano to ten degrees, the same being indicated by the pointer on the phorometer’s scale of measurements. As a guide in dark-room testing, it should be noted that the handle of the phorometer in a vertical position is an indication that the vertical muscles are being tested; if horizontal, the horizontal muscles are undergoing the test. THE MADDOX ROD The Maddox rod (Fig. 14) consists of a number of red or white rods, which cause a corresponding colored streak to be seen by the patient. This rod is placed most conveniently on the instrument, being provided with independent stops for accurately setting the rods at 90 or 180 degree positions. The Maddox rod has proven of valuable assistance in detecting muscular defects, particularly when used in conjunction with the phorometer. Thus employed, it enables the patient to determine when the streak seen with one eye crosses through the muscle-testing spot-light observable by the other eye, as hereafter described. [Illustration: Fig. 14—The Maddox Rod, a valuable aid in making muscular tests.] PROCEDURE FOR MAKING THE MUSCLE TEST The Ski-optometer should be equipped with _two_ Maddox rods, one red and one white. Their combined use is of the utmost importance since they assist in accurately determining cyclophoria and its degree of tortion as designated on the degree scale, and fully described in a later chapter. When the Maddox rods are placed in a vertical position, it is an indication that the vertical muscles are being tested; when placed horizontally, the horizontal muscles are being tested. It should be particularly noted that the streaks of light observable through the Maddox rods always appear at right angles to the position in which they lie.