On the Adjustment of a Spectrometer

The lines of collimation, or axes of the telescope and collimator should lie in one plane, and be always perpendicular to the axis about which the telescope rotates. To secure absolute accuracy in this is a complicated problem.

In practice it is usually sufficient to assume that the axes of the telescope and collimator are parallel to the cylindrical tubes which carry the lenses. Level the table of the instrument by means of a spirit-level and the levelling screws, afterwards level separately the telescope and collimator by means of a level and the set screws attached to each. Then clearly the axis of each is parallel to the plane of the circle.

See that the clamp and tangent screw work properly, and that the instrument is so placed that it will be possible to read the vernier in all positions in which it is likely to be required.

Focus the eye-piece of the telescope on the cross-wires or needle-point turn the telescope to some very distant object, and focus the object-glass by the parallax method described on p. 289. Turn the telescope to look into the collimator; illuminate the slit, and then focus it by altering its position with reference to the lens of the collimator. When the slit is in focus, the light issuing from the collimator forms a series of pencils of parallel rays. This is a very important adjustment; if it be properly carried out the direction of the rays forming an image after reflexion or refraction at the surface of a prism, and hence the circle readings, will be the same, no matter to what extent the prism may be moved parallel to itself about the spectrometer table.

In experiments in which a prism is used it is generally necessary that the edge of the prism should be parallel to the axis of rotation of the telescope. Turn the telescope to view the slit directly. Fix by means of soft wax a hair or silk fibre across the slit, so that it may appear to coincide with the horizontal cross-wire or point of the needle when seen through the instrument; or, as is often more convenient, cover up part of the slit, making the junction of the covered and uncovered portions coincide with the horizontal wire. Fix the prism with wax or cement on to the levelling table in the centre of the instrument, so that the light from the collimator is reflected from two of its faces, and adjust it by hand, so that the two reflected images of the slit can be brought in turn into the field of view of the telescope. Alter the set screws of the levelling table until the image of the hair across the slit when reflected from either of the two faces, and seen through the telescope, coincides with the intersection of the cross-wires. When this is the case the prism is in the required position.

The edge of the prism may also be adjusted to be parallel to the axis of rotation by setting the two faces successively at right angles to the line of collimation of the telescope. This may be done with great accuracy by the following optical method. Illuminate the cross-wires of the telescope, and adjust the face of the prism so that a reflected image of the cross-wires is seen in the field of view of the telescope coincident with the wires themselves. This can only be the case when the pencil of light from the intersection of the wires is rendered parallel by refraction at the object-glass of the telescope, and reflected normally by the face of the prism, so that each ray returns along its own path. An aperture is provided in the eye-piece tubes of some instruments for the purpose of illuminating the wires; in the absence of any such provision, a piece of plane glass, placed at a suitable angle in front of the eye-piece, may be used. It is sometimes difficult to catch sight of the reflected image in the first instance, and it is generally advisable, in consequence, to make a rough adjustment with the eyepiece removed, using a lens of low magnifying power instead.

When fixing the prism on to the table, it is best to take care that one face of the prism is perpendicular to the line joining two of the set screws of the levelling table. Level this face first. The second face can then be adjusted by simply altering the third screw, which will not disturb the first face. It is well to place the prism so that the light used passes as nearly as possible through the central portion of the object-glasses of the collimator and telescope.