- 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
- 94. THE SPECTROMETER.
- 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
- 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
- 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)
- (a) Adjust the eyepiece, telescope and collimator
of the spectrometer using the methods outlined above.
- © R. Paselk
- Last modified 22 July 2000