The earliest picture of a spectrometer I have found is this cut from Hauksbee and Whiston's Course of Lectures, ca. 1705. 

   The liquid-filled glass prism was fixed in position, and the light admitted through the cross-shaped aperture on the left arm. The eye looks through a sighting tube on the end of the right-hand arm. Provision is made for slow-motion drive screws at I and K

A. Two-arm Spectrometers
   The basic spectrometer has a light source S illuminating a slit that acts as an object for lens C. This produces a parallel beam of light illuminating the prism P. After refraction by the prism, the light is focussed by lens O on cross-hairs R. The eyepiece lens E is then used to examine the various images of the slit in the various colors present in the source. The cut is from Wm. S. Franklin and Barry MacNutt, Light and Sound (Bethlehem, PA, 1909)
   This large, unmarked spectrometer is at Denison University in Granville, Ohio. It dates from ca. 1900, and appears to have been equipped with a holder for a diffraction grating at some later date. Student-type spectrometers like this one have been in common use since 1900, although computer-based devices are beginning to take their place. Verniers allow the angular positions of the arm to be read to the nearest minute or two. 
   This incomplete spectrometer at Denison is by John Browning of London. The prism clamp can be clearly seen and the collimating arm exists, but the telescope has been lost over the years. This is not surprising: eyepieces and small reading telescopes were useful adjuncts around a physics department, and were often borrowed. Many physics departments used to have a small cardboard box filled with eyepieces that had come adrift from their original apparatus.

   This instrument is listed at $38 in the 1888 Queen Catalogue of Instruments for Physical Optics; the cut clearly shows how the arms are threaded into their holders. 

   This small, unmarked spectroscope at Kenyon College in Gambier, Ohio has lost its eyepiece. The telescope tube can be adjusted through a small angular range with a tangent screw. 

   The spectrometer below is in the Museum at St. Patrick's College at Maynooth, County Kildare, Ireland. It was made by Adam Hilger of London, stands on its own trolley, and the divided circle has a diameter of 45.7 cm. It was used by Walter Hartley (1846-1913) for his research on the spectra of elements. In 1883 he established that the relationships exist between the wavelengths in the spectrum of an element and its position in the periodic table.

   This Max Kohl spectroscope at the University of Vermont rather resembles Jamin's Divided Circle in its overall form. The 1912 Kohl catalogue that I consulted at Vermont noted that 'the Goniometer can be used horizontally and vertically. It is used for more accurate experiments on reflection, refraction and colour-dissipation, for determining the angles of prisms and refractive indices by Fraunhofer's, Meyerstein's or Listing-Abbe's Method, as a Goniometer and Spectrum Apparatus. Height, 50 cm;  diameter of circle, 55 cm. The apparatus is graduated in two degrees." The price was £18, corresponding to about $100.
   The spectrometer on the right is in the Birr Science Centre on loan from the Galway physics department. 

   The spectrometer was made by Max Kohl of Chemnitz, Germany, and came equipped with a 14438 line per inch diffraction grating ruled by Rowland of Johns Hopkins University on a blank figured by Brashear. 

   The apparatus is listed at 725 marks in the Kohl catalogue published ca. 1900.

   The two-arm spectrometer at the left is in the apparatus collection at Hobart and William Smith Colleges in Geneva, New York.

   Inside the light-tight cover is a diffraction grating.

   It is unmarked, but the style of the feet suggests that it was made about 1900.

. Three-Arm Spectrometers
  "To study spectra... physicists employ instruments called spectroscopes. [The cut at the left] represents one of these. The flame of a gas lamp is placed in the axis of a lens to which the light penetrates through a narrow slit; the slit and lens forming what is called the collimator. The slit being in the focus of the lens, the light passes through the prism in a parallel beam. The light which passes through the refractive medium is made to work an image of the slit is made to form an image of the slit at the focus of another lens, which image is examined by an eyepiece. This arrangement, which is a great improvement on that adopted by Fraunhofer, is due to an English optician of great celebrity, Mr. Simms." (From Amédée Guillemin, The Forces of Nature, Third Edition, trans. by Mrs. Norman Lockyer (MacMillan and Co., London, 1877), pp 327-328
   A present-day user of a spectrometer employs a diffraction grating; if the grating spacing of the grating is known with precision the wavelengths may be obtained directly from the angle at which the particular lines appear.

   For most of the nineteenth century the typical spectroscope used a prism to separate the spectral lines. Calibration was obtained by projecting the lines of a known spectral source onto the same plane as the unknown lines. 

   The diagram at the right, from Franklin and MacNutt, 
pg 129, shows how the light from the reference source (at the lower left of the diagram, is sent into the refracting prism using  total internal reflection from a small prism. To use interpolation to find the unknown wavelengths, a scale AB, etched on a glass reticule, is reflected  from the surface of the prism and projected on the plane RY of the images of the known and unknown sources. A divided circle is not necessary for finding wavelengths.

   The Queen spectrometer at the right in in the collection of Kenyon College in Gambier, Ohio. Apart from a small difference in the top of the column, it appears to be close to Queen's New Laboratory Spectroscope, listed at $35.00 in the 1888 Queen Catalogue of Instruments for Physical Optics. 

   The catalogue notes that "We believe that this spectroscope supplies a want long felt, and that it will be of great service for student's use to save larger and finer instruments in the laboratories of many universities. The extremely low price at which it is furnished brings it within the reach of all."

   The two spectrometers below are at the University of Vermont. The one at the right is described in the 1888 Queen Catalogue of Instruments for Physical Optics as "Bunsen's Laboratory Spectroscope. With One prism. Consists of a collimator, with adjustable slit and a prism for comparison of spectra, a second collimator, with a photographed millimeter scale, and a telescope for examining the rays from the former two. The prism, which is of flint-glass, is inclosed in a strong metal box. All mounted upon a neat stand and packed in a strong box with lock ... $55.00"
   At the left is a Horizontal Spectroscope by Duboscq, probably Cat. No. 179 in the 1885 Duboscq catalogue, where it is listed at 300 francs ($60.00). Note the gas-burner for illuminating the scale.

   The Queen spectrometer at the left is approximately the same as the one at the right above.

It is at Westminster College in western Pennyslvania. It has suffered the fate of many older spectrometers and has lost its eyepiece.

   The unmarked spectrometer at the left, below, is in the collection of Transylvania University in Lexington, Kentucky. At the right is a Societie Genovoise spectrometer at Washington and Jefferson College in Washington, Pennsylvania

  The spectrometer below was at the Science Centre at Birr Castle in Birr, County Offaly, Ireland when I photographed it in September 2000. A year earlier I had seen it at the collection at the physics department at the National University of Ireland, Galway. It is described as "Instrument similar to Original Bunsen and Kirchhoff spectroscope -- Mahogany trapezoid with three brass legs. Housing has hinged lid and both are blackened on the inside; circular prism table, revolved with brass handle below; brass tapering cylindrical telescope and collimator; slit adjusted with small knob -- three holes on housing for reference prism..." in Charles Mollan, Irish National Inventory of Historic Scientific Instruments (Samton Limited, Dublin, 1995), pg 362. The third arm, used to project the image of the linear scale, projects to the rear. A hollow glass prism for holding liquid samples is to its left.

C. One Arm Spectrometers (Direct-Vision Spectrometers)
This direct-vision spectrometer at St. Mary's College in Notre Dame, Indiana, is by John Browning of London. 

   The cut below, from pg 134 of Franklin and MacNutt, shows the arrangement of two crown glass and one flint glass equilateral prisms. This provides zero refraction of the middle part of the prism, but does give dispersion to produce the spectrum. The images of slit S are located at RV, the plane on which the eyepiece E is focussed. There may also a graduated reticule at this point. 

   The side-arm on the spectroscope at the left is probably designed to project an image of a reticule on plane RV.

D. Multiple-Prism Spectrometers
  The two-prism spectrometer by John Browning of London produces twice the dispersion as one using a single prism. The 1888 Queen Catalogue of Instruments for Physics Optics lists this spectrometer at $78, and notes that the two prisms are of "extra dense glass." 

   This instrument is in the Garland Collection of Classic Physics Apparatus at Vanderbilt University, and was probably purchased ca. 1875. 

D. Wavelength Spectrometers

  The  spectrometer at the left is a wavelength spectrometer; the rotating dial in the middle allows one to dial up a specific wavelength. Exit and entrance slits are controlled by the rotating rings on the ends of the instrument. Inside is an equilateral, triangular prism; from the relatively gentle bend in the case it can be seen that the deviation of the incoming light is relatively small.

   The instrument was made by Billingham and Stanley, Ltd. of London, and is in the Greenslade Collection.

E. Spectrometers for Telescopes
   The spectrometer on the left was built by John A. Brashear of Allegeny, Pennsylvania, for the Smith Observatory at Hobart College. The date is 1888. The device at the bottom was used to attach the spectrometer to the telescope: the ring clamped around the telescope tube and the tubes above and below the collimator arm on the right-hand side slipped into the two rods on the connector. The spectrometer can also be used off the telescope.

   Brashear (1840-1910) was a self-taught astronometer and instrument maker who produced, among other projects, the 72 inch mirror for the Dominion Observatory in Victoria, British Columbia.

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