The College of Optometrists

Newton's optical legacy was particularly profound in the field of spectroscopy. In 1666 Newton discovered that white light was made up of the basic colours of the spectrum. This disproved the earlier belief that colours were caused through white light being somehow modified. The sun, however, is not the only source of celestial light. Thus, in the late eighteenth century, William Herschel experimented with the spectra produced by the light from other stars. In 1783 he placed a prism at the eyepiece end of a 20-foot reflecting telescope and in 1798 he investigated light from six bright stars including Sirius and Arcturus.

 

In 1802 William Hyde Wollaston (1766-1828) refined Newton's prism experiment by reducing the light source to a narrow slit 1/20th inch wide. He noted dark lines across the spectrum and at first assumed that these were the divisions between the seven colours.

 

Joseph Fraunhofer (1787-1826) discovered hundreds of similar lines whilst testing glass telescope lenses. Eventually he counted 576. Fraunhofer had gained experience as an optician working at the Utzschneider optical institute near Munich. He also reproduced spectra in laboratory conditions that differed from that of sunlight in not being continuous ('bright-line' spectra, with dark gaps between the lines).

 

Gustav Robert Kirchhoff (1824-1887) explained Fraunhofer's discoveries in 1859. He established that sunlight is just one of a number of glowing forms of matter (liquid or solid) that produce a continuous spectrum; bright-line spectra are caused by gases. Each element, whether found in gaseous, liquid or solid form could be analysed and a different tell-tale set of line positions identified. It was now possible to determine the chemical composition of celestial bodies and this gave way to a new science of astro-physics.

 

 

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Some relevant instruments from the BOA Museum collection....

 

  

1999.327 Spectrometer by Philip Harris Ltd. Black metal and brass with a tripod base. Contained in wooden case with a hinged door that opens at the front. English (Birmingham).

1999.3515 Grace's Spectroscope, made by John Browning, 63 Strand. Brass cylindrical body with screw lenses, milled adjusting wheel. English (London). Late 19th c.

 

This is a diffraction spectroscope. John Browning's Prismatic Instruments Catalogue (March 1923) states:

This instrument was originally designed for meteorological use and has for many years been known as Grace's Rainband Spectroscope, but is equally suitable for all the various purposes for which Direct Vision Spectroscopes can be used. It has greater dispersion than the miniature type and the mechanical focusing adjustment will be found useful for faint lines and bands as distinct from the well-marked absorption bands, for which the simpler patterns of the Miniature type suffice.

 

John Browning was noted for his fine spectroscopes though he also sold a wide range of other optical equipment. He was also the first President of the British Optical Association 1895-1900.

 

1999.3516 Rain-band spectroscope & case, made by John Browning, 63 Strand. Brass cylindrical body with telescopic tube and screw lenses. Contained in cylindrical cardboard case with slip top and no end pieces. English (London). Late 19th c. It's quite short. That's a five-centimetre scale shown next to it.

 

A rain band spectroscope was for viewing the broad red band that raindrops produce in a spectrum; these were popular items, not necessarily for serious scientists. Note the wording of this advertisement for a slightly earlier instrument from an 1861 issue of Chemical News:

 

Crookes' Spectroscope...with the kind and valuable assistance of W. Crookes, Esqu. F.C.S. so well known for his successful researches upon the spectrum, we have been enabled to produce a most efficient, portable and convenient instrument...Crookes' Pocket Spectroscope for tourists...Spencer Browning and Co., Patentees, 111 Minories, E.C. Established 100 years.

 

1999.2761 Thorp's Spectroscope & case, by R. & J. Beck Ltd. Brass D.V. Diffraction Rendering Spectroscope, cylindrical, with adjusting wheel and brass screw-cap on end. Also a tiny round lens. Contained within black leatherette hinged case with clasp, lined with blue velvet and purple silk. English (London).