Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

The following instructions have been taken, with minor editing, from: Minor, Ralph S. Physical Measurements: A Laboratory Manual in General Physics for Colleges. Part IV: Sound and Light. Berkeley, Calif. (1917) pp. 24-26.
Copyright © 1998 Richard A. Paselk

The spectrometer consists of a graduated circle, generally fixed in a horizontal position, to which is attached a collimator and a telescope. The collimator consists of a tube containing an achromatic lens at one end and a vertical slit at the principal focus of the lens. The movable arm carries an astronomical telescope which is always directed towards the center of the graduated circle. The position of the telescope with reference to the fixed circle may be read by means of a vernier. Above the center of the graduated circle is a horizontal table, called the table of the spectrometer, which is capable of rotation about the vertical axis of the circle. The position of the circle table can be determined by means of the vernier.
Adjustment of the Spectrometer, Using a Gauss Eyepiece.
1. The Eyepiece. The eyepiece should be moved in or out until the cross-hairs are distinctly seen.
2. The Telescope. As the adjustment secured by focusing the telescope on a distant object is only approximate the following method of focusing the telescope for parallel light should be used. Cover the objective of the telescope with a plane mirror. Place a light at right angles to the axis of the telescope opposite the opening in the Gauss eyepiece so that the cross-hairs will be illuminated and the glass plate, inclined at 45° to the axis of the tube, sends some light down the barrel of the telescope. Move the tube carrying the eye-piece and cross-hairs until there is no parallax between the cross-hairs and their image, formed by the light reflected from the plane mirror. The cross-hairs are then in the focal plane of the telescope.
Set the cross-hairs at 45° with the vertical.
3. The Collimator. Illuminate the slit, turn the adjusted telescope into line with the collimator and then, while looking through the telescope, move the slit in or out till there is no parallax between its image and the cross-hairs. The slit is then in the focal plane of the collimator lens.
This is an important adjustment since when the slit is in focus the light coming from the collimator has a plane wave front, and the curvature of the wave-front will not be altered by the introduction of the prism or by any lateral displacement of the prism.
After the above adjustments have been made, if there is any difficulty in seeing the cross-hairs, the eyepiece may be moved, but not the cross-hairs themselves.
For work of extreme accuracy the axes of the telescope and collimator must lie in one plane, and always be perpendicular to the axis about which the telescope rotates, and the faces of a prism or the plane of a grating placed on the spectrometer table should be parallel to this axis.
These adjustments require patient and careful manipulation, and in the following work with the spectrometer the student may assume that they have been made. The methods of adjustment which have been used are given below.
To adjust the Telescope perpendicular to the Axis of the Spectrometer: Use a Gauss eyepiece, illuminating the cross-hairs with some convenient source of light. Place a plane-parallel plate of glass upon the table of the spectrometer with one edge parallel to any two leveling screws (G F. See Figure). Turn the table so that the glass reflects light back down the telescope and adjust the table until the reflected image of the cross-hairs is seen in the field of view. If the cross-hairs do not coincide vertically with their image correct half the distance by adjusting the table, by means of leveling screw H, the other half by moving the telescope. Rotate the table 180° until the reflected image is again in the field. If the two images coincide for both positions of the table, the telescope is perpendicular to the axis of the instrument and the plane of the glass plate is parallel to this axis.
To adjust the Collimator perpendicular to the Axis of the Spectrometer: Illuminate the slit and rotate the telescope until the image of the slit is seen in the telescope, adjust the collimator until the image of the cross-hairs on the collimator slit coincide with the intersection of the cross-hairs in the field of the telescope. The collimator is then perpendicular to the axis of the instrument.
To adjust the Faces of the Prism parallel to the Axis of the Spectrometer: Set the prism on the spectrometer table with the face A C at right angles to a line through F G. The inclination of A C may be changed by turning either F or G. Using the Gauss eye-piece as directed above adjust A C until the cross-hairs coincide with their image in the field of view.
Raising or lowering H will simply move the face A C in its own plane so that if F and G are left adjusted the face A C may be adjusted by turning H until the reflected image from this face also coincides with the cross-hairs. Adjust the face A B in this manner. All three faces of the prism are then vertical, that is, parallel to the axis about which the telescope (or the spectrometer table) rotates.
(a) Adjust the eyepiece, telescope and collimator of the spectrometer using the methods outlined above.

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Last modified 22 July 2000