While working on certain changes in the tissues of the guinea-pig, it proved of great advantage, in observing the finer changes, to use two microscopes side by side, one showing the normal tissues and the one the abnormal tissues. On Jan. 29, 1912, I communicated with Mr. Edward Bausch, suggesting that a microscope be so equipped with two objectives, "that on looking through the eyepiece one would see half of the field of each objective." The Bausch and Lomb Optical Company kindly worked out the idea, using, however, two microscopes and one comparison ocular, which method is more practical and less expensive than that of two objectives on one microscope.