In the last the field is full of false light, and it is not possible to give sufficiently minute and steady separation to the images; and there are of necessity a collimator, two prisms of total reflection, and a small telescope through which the rays must pass; consequently there is great loss of light.
Spectroscopes generally consist of three parts: (I) the collimator; (2) the analysing appliance, (3) the telescope.
The slit of the collimator confines the light to a nearly linear source, the beam diverging from each point of the source being subsequently made parallel by means of a lens.
The lens may then be also dispensed with, and the whole collimator becomes unnecessary if the luminous source is narrow and at a great distance, as for instance in the case of the crescent of the sun near the second and third contact of a total solar eclipse.
The general results may be summarized as follows: if the width of the slit is equal to fX/4D (where X is the wave-length concerned, D the diameter of the collimator lens, and f its focal length) practically full resolving power is obtained and a further narrowing of the slit would lead to loss of light without corresponding gain.
Theoretical resolving power can only be obtained when the whole collimator is filled with light and further (as pointed out by Lord Rayleigh in the course of discussion during a meeting of the " Optical Convention " in London, 1905) each portion of the collimator must be illuminated by each portion of the luminous source.
The collimator of a spectroscope should be detached, or moved so as to admit of the introduction of an auxiliary slit at a distance from the collimator lens equal to its focal length.
If a source of light be placed behind the auxiliary slit a parallel beam of light will pass within the collimator and fall on the slit the width of which is to be measured.
With fairly homogeneous light the diffraction pattern may be observed at a distance, varying with the width of the slit from about the length of the collimator to one quarter of that length.
The stand carries three tubes: the collimator, observing telescope and scale telescope.
The collimator has a vertical slit at its outer end, the width of which may be regulated by a micrometer screw; in some instruments one half of the slit is covered by a small total reflection prism which permits the examination of two spectra simultaneously.
At the other end of the collimator there is a condensing lens for bringing the rays into parallelism.
In grating spectroscopes both plane and concave gratings are employed in connexion with a collimator and observing telescope.
Suppose a fixed image of the sun to be formed on the collimator slit of this spectroscope, and a photographic plate, with its plane parallel to the plane of the solar image, to be mounted almost in contact with the camera slit.
The spectroscope is then moved parallel to itself, admitting to the collimator slit light from all parts of the sun's disk.
The platform carries the two slits, the collimator and camera objectives and the prism-train.
In diameter, is formed by the Snow telescope on the collimator slit (d).
Collimator objective (e), which is constructed in the manner of a portrait lens in order to give a sharp field of sufficient diameter to include the entire solar image.
In diameter on the collimator objective, as its focal length is 60 in.
The rays, rendered parallel by the collimator objective, meet a plane mirror (f) of silvered glass, which reflects them to the prisms (g, g').
Collimator of long focus.
Attempts have been made to overcome the difficulties connected with levels by adopting the principle of Kater's floating collimator (Phil.