From the Cabinet of Physics: Better and Better Spectra

In the past, I remarked on the Cabinet of Physics demonstration using a single-prism spectroscope to view the continuous spectrum of a hot carbon-arc light source.

Christoper Graney compared this demonstration video to another where a spectroscope reveals “emission lines” in gases heated by electrical discharges.

In today’s demonstration, we examine an improved spectroscope, comparing it to the single-prism spectroscope we have seen before. As the nineteenth century marched on, instrument-makers developed devices more precise and easier to use. Here we have a specially-shaped prism, mounted on a turntable, which gives greater dispersion to the incoming light and “stretches out” the spectrum, so that it is possible to see more detail.

In the eyepiece of the older spectroscope, the entire spectrum of light from a copper arc is visible: a continuous glow from the heated vapor, punctuated with the bright emission lines characteristic of copper.

The fancier instrument dispays only a portion of the spectrum. As the user turns a drum that rotates the prism on its turntable, we tour the spectrum. Colors parade past; first red, then orange, then green, then blue. The relative position and brightness of copper’s glowing emission lines can be seen more clearly. And markings on the drum allow the user to read off the wavelengths of features in the field of view.

One can readily imagine that quantitative work, to measure and compare spectra of various substances, is made easier by such a spectroscope. It became a valuable tool for analytical chemistry. As the technique of spectrocopy developed, as we know, it was to pave the way for new discoveries in astronomy as well.

Here’s another demonstration of an advanced spectroscope.

This one uses four triangular prisms which can rotate in a synchronized dance to produce a wide dispersion of colors. The demonstration gives a good look at the famous pair of yellow lines in the spectrum of sodium. This instrument also superimposes an illuminated scale on the field of view, so that the wavelength of a spectral feature may conveniently be read. I note that this display also advertises its maker, the instrument-shop of Jules Duboscq in Paris—which was also the source of other equipment to the Cabinet's collection.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

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