1. A downhole spectrometer comprising: a sonde for traversing a borehole; an instrument body in the sonde comprising: at least one interference filter attached to a spindle so that, upon rotation, the angle at which light strikes the filter changes; at least one light-detecting device associated with at least one interference filter; a collimated light beam incident on a fluid sample, a portion of light incident upon the fluid sample and incident upon the face of at least one interference filter, and incident upon the light-detecting device with which it is associated; a rotator device to rotate the spindle and its attached at least one interference filter relative to the incident light upon said at least one interference filters; and a processor for determining a spectral component for the sample.