Polariscope or Polarimeter is an instrument for measuring the amount of rotation produced by a substance on the plane of polarisation of plane polarised light. In its simplest, form it would consist of two Nicol's prisms (q.v.); parallel light sent through the first or "polariser" would be polarised in one plane; it then passes through a second Nicol or "analyser." An observer looking through the analyser will have the field of view variously illuminated according to its position. By rotating this Nicol round an axis perpendicular to its plane a point can be reached when all appears dark - i.e. the incident light is completely quenched. If now an "active" substance be put between polariser and analyser, more or less light will get through; by again rotating the analyser the dark position can be rediscovered. The amount of rotation measures the angle through which the active substance has rotated the plane of polarisation. Since it is not easy to be quite certain when the position of absolute darkness is obtained, various devices have been used to avoid the difficulty. The field of view is divided into two parts, and some arrangement is used so that the planes of polarisation of the light are slightly inclined to each other in the two halves. In one case this effect is produced by dividing the analyser into two parts, which are connected together after one of them has been turned end for end. The observer, in using this analyser, rotates it until the two halves are equally dark. Another instrument is provided with what is known as a "biquartz," the two parts of which turn the plane of polarisation in opposite directions; in this arrangement the two parts of the field are brightly coloured, the hues changing as the analysing Nicol's prism is rotated. A position is obtained when the colours of the two halves of the field are the same. Not only crystalline solids have the power of rotating the plane of polarisation of light, but solutions of many substances have the same property, and the amount of rotation produced by a solution in a tube of definite length is often used to determine the amount of active substance present.