=67. Interchangeable Instruments.=—Some of the instruments in common use are arranged to be used either with ordinary lamp or gas light, or with a monochromatic flame. Laurent’s polarimeter is one of this kind. The compensating instruments also may have the field of vision arranged for tints or shadows. Theoretically the best instrument would be one in which the light is purely monochromatic, the field of vision a shadow, and the compensation secured by the rotation of the second nicol. The accuracy of an instrument depends, however, on the skill and care with which it is constructed and used. With quartz wedges properly ground and mounted, and with ordinary white light, polariscopes may be obtained which give readings as accurate as can be desired. Since many persons are more or less affected with color-blindness, the shadow are to be preferred to the tint fields of vision. For practical use in sugar analysis the white light is much more convenient than the monochromatic light. For purposes of general investigation the polarimeters built on the model of the laurent are to be preferred to all others. Such instruments are not only provided with a scale which shows the percentage of sucrose in a solution, but also with a scale and vernier by means of which the angular rotation which the plane of vibration has suffered, can be accurately measured in more than one-quarter of the circle. DESCRIPTION OF POLARIZING INSTRUMENTS. =68. Rotation Instruments.=—This instrument has already been described as one in which the extent of deviation in the plane of polarized light caused by the intervention of an optically active substance is measured by rotating one of the nicols about its axis and measuring the degree of this rotation by a vernier on a graduated arc. In Germany these instruments are called _polaristrobometers_, and in France _polarimètrés_. In England and this country the term _polariscope_ or _polarimeter_ is applied without discrimination to all kinds of optical saccharimeters. The polariscope of Mitscherlich was one of the earliest in use. It has now been entirely superseded by more modern and accurate instruments.