=ACETYL.= _Syn._ ACETYLE. A name originally given to a hypothetical body, having the formula C_{2}H_{3}, and regarded by Berzelius as the radical of the acetates and their congeners. The acetyl of Gerhardt (C_{2}H_{3}O) is, however, according to that chemist, the true radical of the acetates. Williamson, in order to remove the confusion of terms occasioned by the application of the same name to compounds of different composition, proposed the title of othyl for the radical C_{2}H_{3}O. =ACHAR.= See PICKLES. =ACEIILE′INE= (-kĭl-). A peculiar bitter principle obtained from achillé a millefolium (Linn.), or yarrow. =A′CHOR=, (-kŏr). [Gr.] See SCALD-HEAD. =ACHROMAT′IC= (ăk-ro-). _Syn._ ACHROMATIQUE, Fr. In _optics_, devoid of colour; bodies that transmit light without decomposition, and consequently, without the formation of coloured rings or fringes; applied to compound lenses, prisms, &c., and to instruments fitted with them. =ACRO′MATISM.= _Syn._ ACHROMATISME, Fr. In _optics_, the state of being achromatic; the absence of coloured fringes in the images of objects seen through a lens or prism. Light is not homogeneous, but decomposable by refraction, absorption, or reflection, into coloured rays of unequal refrangibility. A ray of white light, in passing through a glass prism, is entirely separated into the coloured rays forming the ‘prismatic spectrum,’ and when it passes through a lens, an analogous resolution into coloured rays still occurs, though not so readily observed, and that to an extent often incompatible with distinct vision. Now, if a convex lens be regarded as a number of prisms united by their bases round a common centre, and a concave lens, as a similar number of prisms with their apices in contact, the action of lenticular and prismatic glasses on light will be reduced to a common principle. A beam of light thrown on a simple converging lens not only suffers refraction at the spherical surface (SPHERICAL ABERRATION), but the different coloured rays of which it is composed, from the causes mentioned, being unequally bent or refracted, diverge from their original course (CHROMATIC ABERRATION), forming as many foci on the axis of the lens as there are colours, and fall separately, instead of together, on the eye or object which receives them. Hence arise the coloured fringes or halos that surround objects viewed through ordinary glasses, and which form the great impediments to the construction of perfect lenses. This effect, like the refractive power and focal distance, varies in degree in different diaphanous substances.