The Probert Encyclopaedia of Science & Technology

SPECTROSCOPE

A spectroscope is a mechanical device for analysing light. The Spectroscope resolves light into vibrations of different frequencies, so that its properties can be defined. Spectroscopes are used for such things as measuring the velocity of stars, looking at the rotation of the sun and the detection of chemical elements.
Mechanical spectroscopes generally included a slit and a collimator to admit the light in a parallel beam, and a viewing telescope. The Littrow type combines collimator and telescope, making the beam pass twice through the same lens. With a camera replacing the eyepiece, the instrument becomes a spectrograph, and when equipped with measuring scales and circles, a spectrometer.
The actual analysis is effected in refracting spectroscopes by one or more prisms of glass, or other refracting medium, which, by causing rays of shorter wave-lengths to deviate more than longer ones, splits up the beam into a rainbow spectrum with the red rays nearest the thin edge of the prism. As the dispersion increases with diminishing wave-length, the violet end is spread out more than the red, and the dispersion is called irrational. Small direct-vision instruments are made with an odd number of prisms in one tube, the even numbers reversed and of denser glass, so that for some mean ray the deviations cancel each other, the instrument being used pointing directly towards the light.
For some astronomical purposes the collimator is unnecessary, and the prism can be fixed outside the object glass of a telescope (known as an objective prism). For very refined measurements the resolving power of a prism is insufficient and a diffraction grating was used. Fraunhofer first tried an actual grating of fine silver wire, and afterwards an optical grating of parallel lines rules by a diamond on a glass plate. Later, silvered glass was used for reflection instead of transmission, and then speculum metal replaced the glass.
Diffraction spectra are formed in sets, first, second, third order etc, on each side of the directly reflected rays, with the violet end nearest the central undisturbed image. An idea of them may obtained by looking at the sun through a feather. Ruled gratings being very expensive, cheap replicas, called Thorp gratings, were made by moulding melted celluloid on a ruled grating. Rowland's concave grating acts as its own condenser and focusing lens, thus avoiding the loss of light due to absorption. It yields a perfectly normal spectrum when used in certain positions.
Resolving power is the ratio of the wave-length to the smallest difference of wave-length actually separated by the instrument. With a very narrow slit it nearly equals the number of lines in the whole grating multiplied by the order of spectrum considered. Michelson produced a grating with a resolving power of 300,000. He also invented a new form of optical grating called an echelon, comprised of glass plates of uniform thickness being arranged in steps. Higher resolving power is reached by interferometers, especially Michelson's. In these the analysis is produced by passing the ray between parallel plates of glass, one or both only partially silvered, the phase of emergent rays varying with the number of internal reflections.
Research Spectroscope