It is said that Prof. Eliot of Edinburgh in 1834 was the first to conceive of the idea of a stereoscope, by which two different pictures of the same object, taken by photography, to correspond to the two different positions of an object as viewed by the two eyes, are combined into one view by two reflecting mirrors set at an angle of about 45°, and conveying to the eyes a single reflection of the object as a solid body. But Sir Charles Wheaton in 1838 constructed the first instrument, and in 1849 Brewster introduced the present form of lenticular lenses. Brewster also demonstrated the utility of dioptric lenses, and zones in lighthouse illumination; and in which field Faraday and Tyndall also subsequently worked with the addition of electrical appliances. The labours of these three men have illuminated the wildest waters of the sea and preserved a thousand fleets of commerce and of war from awful shipwreck. As illustrating the difficulties sometimes encountered in introducing an invention into use, the American Journal of Chemistry some years ago related that the Abbé Moigno, in introducing the stereoscope to the savants of France, first took it to Arago, but Arago had a defect of vision which made him see double, and he could only see in it a medley of four pictures; then the Abbé went to Savart, but unfortunately Savart had but one eye and was quite incapable of appreciating the thing. Then Becquerel was next visited, but he was nearly blind and could see nothing in the new optical toy. Not discouraged, the Abbé then called upon Puillet of the Conservatoire des Arts et Metiers. Puillet was much interested, but he was troubled with a squint which presented to his anxious gaze but a blurred mixture of images. Lastly Brot was tried. Brot believed in the corpuscular theory of light, and was opposed to the undulatory theory, and the good Abbé not being able to assure him that the instrument did not contradict his theory, Brot refused to have anything to do with it. In spite, however, of the physical disabilities of scientists, the stereoscope finally made its way in France. Besides increasing the power of the eye to discover the secrets and beauties of nature, modern invention has turned upon the eye itself and displayed the wonders existing there, behind its dark glass doors. It was Helmholtz who in 1851 described his _Ophthalmoscope_. He arranged a candle so that its rays of light, falling on an inclined reflector, were thrown through the pupil of the patient’s eye, whose retina reflected the image received on the retina back to the mirror where it could be viewed by the observer. This image was the background of the eye, and its delicate blood vessels and tissues could thus be observed. This instrument was improved and it gave rise to the contrivance of many delicate surgical instruments for operating on the eye.