The groove thus formed may be deepened by another acid solution. When thus produced, as many copies of the record as desired may be made by the electrotyper or print plater. The public is now familiar with the different forms of this wonderful instrument, and like the telephone, they no longer seem marvellous. Yet it is only within the age of a youth or a maiden when the allegations or predictions that the human voice would soon be carried over the land, and reproduced across a continent, or be preserved or engraven on tablets and reproduced at pleasure anywhere, in this or any subsequent generation, were themselves regarded as strange messages of dreamers and madmen. _Optical Instruments._--There were practical inventions in optical instruments long before this century. Achromatic and other lenses were known, and the microscope, the telescope and spectacles. The inventive genius of this century in the field of optics has not eclipsed the telescope and microscope of former ages. They were the fruits of the efforts of many ages and of many minds, although Hans Lippersheim of Holland in 1608 appears to have made the first successful instrument “for seeing things at a distance.” Galileo soon thereafter greatly improved and increased its capacity, and was the first to direct it towards the heavens. And as to the microscope, Dr. Lieberkulm, of Berlin, in 1740, made the first successful solar microscope. As well known, it consisted essentially of two lenses and a mirror, by which the sun’s rays are reflected on the first lens, concentrated on the object and further magnified by the second lens. The depths of the stars and the minutest mote that floats in the sun beam reflect the glory of those inventions. The invention of John Dolland of London, about 1758, of the achromatic lens should be borne in mind in connection with telescopes, microscopes, etc. He it was who invented the combination of two lenses, one concave and the other convex, one of flint glass and the other of crown glass, which, refracting in contrary ways, neutralised the dispersion of colour rays and produced a clear, colourless light. Many improvements and discoveries in optics and optical instruments have been made during the century, due to the researches of such scientists as Arago, Brewster, Young, Fresnel, Airy, Hamilton, Lloyd, Cauchy and others, and of the labours of the army of skilled experts and mechanicians who have followed their lead. Sir David Brewster, born in Scotland in 1781, made (1810-1840) many improvements in the construction of the microscope and telescope, invented the kaleidoscope, introduced in the stereoscope the principles and leading features which those beautiful instruments still embody, and rendered it popular among scientists and artists.