Spectroscopy

When produced in this way the spectrum consists of a succes sion of images of the slit, depending upon the nature of the source, and while the slit may be of any shape, a narrow straight slit is ordinarily used because it gives the least confusion of overlapping images. It is because a straight slit is most generally used that we speak of spectrum lines.

When a ray of light leaves the prism at the same angle as that at which it enters, the prism is said to be at minimum deviation for that particular colour or wave-length. This position can read ily be determined by turning the prism until the line under obser vation advances as far as possible in the direction of the red. When a collimator is used, it is not essential for good definition that the prism should be at minimum deviation, but it is usually most convenient to place it in this position, so that a previous condition of adjustment can easily be restored.

Constant Deviation Prism.

A convenient form of prism which is optically equivalent to a 6o° prism of the same kind of glass has been utilized in the Hilger wave-length spectrometer, which is illustrated in fig. 3. The prism takes the form shown in fig. 4, and though made in one piece of glass it may be considered to be made up of two prisms with refracting angles of 30°, acting in conjunction with a totally reflecting prism ABC.

When such a prism is rotated about a vertical axis, different wave-lengths may be observed in a fixed direction at right angles to the incident ray, and a prism of this form is consequently called a constant deviation prism. Measurements are made on a large drum which forms the head of a fine tangent screw by which the prism is rotated, the divisions of the spiral scale being such as to indicate wave-lengths di rectly.

Compound Prisms.

It is often convenient to have an in strument in which the viewing telescope is in direct line with the collimator, and, of the various ways of obtaining this, the most common arrangement is that in which prisms of different kinds of glass are combined to give disper sion without deviation. This is

possible because in different kinds of glass dispersion and deviation are not proportional; flint glass, for example, giving a greater dis persion in proportion to deviation than crown glass. Such a com bination is illustrated in fig. 5a, the central flint prism being sup posed to have a refractive index of 1.751 and the outside crown glass prisms 1.52o. These combinations may give quite a large bal ance of dispersion, because the flint glass prism has a much larger angle than is possible when a prism of the same kind of glass is used alone. Such prisms are used extensively in pocket spectro scopes, which dispense with an observing telescope, as well as in larger instruments. To give increased dispersion, these direct vision prisms are often constructed with two prisms of flint and three of crown glass, cemented together, and two or more such combinations may be used in line with each other. Since direct vision can only be obtained for a particular wave-length, the observing telescope must be capable of movement to permit obser vations of different parts of the spectrum.

Another kind of compound prism, called the Rutherfurd prism, (fig. 5b.) is not designed to give direct vision, though somewhat similar in construction. A flint glass prism of refracting angle about 90° (which could not be used alone) is flanked by two prisms of crown glass with the angles chosen so that the light will pass conveniently through the flint prism. Sometimes, the central prism is replaced by a hollow prism filled with carbon bisulphide.