THE SORBY-BROWNING MICROSPECTROSCOPE

DESCRIPTION:
This Sorby-Browning Microspectroscope is signed: John Browning, London. This instrument comes in its original leather-covered fitted case with a curved top, measuring 9.5(L) X 4.5(W) inches (241 x 114 mm). It has a maximum height of 2 9/16 inches(72 mm). It has a push-button metal clasp holding it closed. The bottom of the inside of the case is lined in blue velour and the top in blue silk. The inside top has a tab on the left to pull the innermost lining, hinged on the right, to reveal a compartment for instructions inside. A set of handwritten instructions, and a diagram are included, both by pencil, but they do not exactly match this instrument exactly and were likely copied from a description. This example is very similar to the original design described in John Browning's How to Work with the Spectroscope, pages 40-43, and 45-47. With the focusing turned all the way down, the instrument measures about 7 1/4 inches(184 MM) in length and extends to 7 3/4 inches (197 mm)when the focusing knob opens the length maximally. The part that fits the microscope tube measures 30 mm (1 3/16 inches) in diameter. The top portion nearest the eye slides in and out of the top tube of the microspectroscope and is easily removeable; it has a pin that guides it in a vertical slot in the top tube of the microspectroscope during rough focusing on the spectrum. The tube itself has a pin that secures it in a L-shaped slot in the bottom half of the microspectroscope so this tube can also be removed easily if needed. Fine focusing on the spectrum is by rack and pinion controlled by the largest knob. Below the knob that opens the main slit of the spectroscope(C below), is a short rod that also extends to the opposite side, which opens and closes the shutter of the accessory prism on the side of the instrument. The plate on that side has V-shaped sides to accept a tube with a liquid reference standard, held in place by spring clips. A gimbaled mirror is attached to an arm on a ball-and-socket joint to facilitate easy positioning to direct light directly through the side slot directly, or through the reference tube.

This engraving from Browning's book illustrates the controls of the instrument:

1. Tube carrying the compound direct-vision prism with arrangement for coarse focusing the spectra
2. Knurled knob for fine focusing the spectra in the eyelens
3. Knurled knob to open or shut the slit vertically
4. Apparatus to hold reference spectra in a tube or on a slide
5. Knurled knob to open and close the slit for the secondary or reference spectrum
6. Field lens in the eyepiece
7. Tube made to fit the main tube of the microscope as would an ordinary eyepiece
8. Knurled knob to regulate the main slit horizontally

Notable differences from the engraving include:
1. A sliding bar(*) beneath 'C' which opens and closes a shutter over the accessory spectrum opening on the side of the instrument replaced the knurled knob control at E.
2. The focusing knob is on the oppositve side of the tube.
3. The arm holding the mirror is now in a more convenient ball-and-cup swiveling mount.

Shown to the left is a schematic drawing representing a cross section of the instrument showing the five main prisms, and the accessory reflecting prism.

Shown to the left is the arrangement of adjustments for the slit and accessory prism. The accessory stage is on the right of the image in this view and light would be directed using the mirror from the right side, through the reference sample, to the accessory reflecting prism, r. The slit is formed between two plates m. and n. n. is a fixed plate and m., acted on by a spring, is moved by the control h to adjust the size of the slit opening. p. is a moveable plate controlled by knob c which regulates the width of the slit and is also acted on by a spring. s. controls the sliding shutter that opens or closes the port from the accessory stage.

HOW THIS INSTRUMENT IS USED(after Browning):
1. Remove A and open the slit by means of H
2. Focus on the slide object in the usual way(using the focus controls of the microscope, not the microspectroscope.
3. Replace A and gradually close the slit using C, until a good spectrum is obtained, which may require the object to be slightly out of focus.
4. Using the focusing control of the microspectoscope B, focus on the part of the spectrum that is of interest. Note that not all parts of the spectrum will be in focus at the same time.
5. When studying crystals, a 1/8 inch cardboard diaphragm below the object is helpful

Among interesting subjects are gelatin filters which if side by side can show their spectra next to each other for comparison.

HISTORY OF THE MICROSPECTROSCOPE:
The first spectroscopes were made by Robert Bunsen and Gustav Kirshoff in Germany in 1859. Prior to that simple prisms were used to study the spectrum. Bunsen discovered the emission lines that characterize certain elements when heated in a flame, and the absorbtion lines of the sun. This enabled identification of elements in the Sun and Stars leading to the discovery of the sequence of the evolution of stars-the new science of astrophysics.

It soon became obvious that the complicated and heavy forms of angled spectroscopes were inconvenient, particularly for travel. In 1860, Geovanni Amici invented a design for a direct vision spectroscope and discussed his idea with Donati. Amici did not publish the design, and Donati was apparently the first to construct it. His manuscript describing it was published in 1862. It used a stack of three equilateral prisms.

Upon hearing about Amici's design, Pierre Jules Cesar Jannsen published a paper about modifications of this design, including the use of up to five prisms. He communicated his ideas to Jean George Hoffman of Paris, who first constructed the Jannsen direct vision spectroscope in 1862. But Jannsen was an astronomer, and the design was, to my knowledge not initially adapted to the microscope. Astronomical, handheld(left) and tabletop models were produced.

In 1865 Alexander Stewart Herschel(son of Sir John), invented another prism that allowed use of a straight tube. But Herschel's design was difficult to make, and likely had poor dispersion.

John Browning improved the design by using two of Herschel's prisms, apparently to improve dispersion. The example shown to the right provided the ability to use one or two of Herschel's prisms. A binocular instrument for studying meteors was apparently made by Browning for Herschel incorporating a single prism on each side.

The adoption of the spectroscope to the microscope, was first accomplished by Henry Clifton Sorby using a simple prism. Soon after, Huggins proposed adoption of the direct vision star spectroscope to the eyepiece and early versions, constructed by John Browning were still relatively large and heavy.

It was Sorby again, who proposed the design of microspectroscope first made by John Browning which became known as the Sorby-Browning Microspectroscope. The original model is shown here to the left. It used a direct vision path of cemented equilateral prisms of alternating flint and crown glass, as in the Jensen-Hoffman design, and provided a side port through which a comparison spectrum could be directed. This instrument is essentially a Huygenian eyepiece with the adjustable slit of the spectroscope positioned between the bottom lens and the eye lens. The prisms are housed in the tube above the eyelens and focusing of the prisms is provided to show the spectrum to best advantage. This became the basis for all microspectroscopes made for next many years.

To the right is shown an example with additional optional features including a calibrated scale of wavelength which could be superimposed on the spectrum being studied and a pointer which moved along the scale controlled by a calibrated dial.