MR. C. D. AHRENS, the veteran prism-cutter, has lately devised a new type of liquid prism, which seems to have some advantages in optical work, both for direct-vision and for ordinary patterns of spectroscope. It is more than fifteen years since Wernicke proposed to employ in a direct-vision combination the highly dispersive liquid cinnamic ether. He found amongst modern sorts of optical glass one kind, a baryta crown, having the same mean refractive index, namely, 1.56, but having only about one-fifth as much relative dispersion. He was therefore able to make a flat-ended direct-vision prism by enclosing a glass prism of from 120° to 130° of refracting angle in a. cell filled with the cinnamic ether, which thus constituted a triple combination, the glass prism being flanked by two reversed prisms of the ether. Several varieties of Wernicke's prism came into favour; but it had the drawback that cinnamic ether is expensive, and for some reason becomes cloudy after standing for a year or two in the containing cell.