Comparison of Spectra

Many spectroscopes are arranged so as to allow the spectra of two distinct sources of light to be examined simultaneously.

To effect this a rectangular prism ABC (fig. 36 [1]) is placed behind the slit of the col-limator in such a way as to cover one half, suppose the lower, of the slit.

Light coming from one side falls normally on the face BC of this prism, and is totally reflected at the face AB emerging normally from the face CA; it then passes through the slit LM and falls on the object glass of the collimator. In some cases a prism of 60° is used (fig. 36 [2]).

The second source of light is placed directly behind the slit and is viewed over the top of the prism.

One half of the field then, the upper, in the telescope is occupied by the spectrum of the light reflected by the prism, while the other is filled by that of the direct light.

We may use this apparatus to compare the spectra of two bodies.

Suppose we have to determine if a given substance contain strontium.

Take two Bunsen burners and place in one a portion of the given substance on a piece of thin platinum foil, while some strontium chloride moistened with hydrochloric acid is placed in the other on a similar piece of foil The two spectra are brought into the field. If the strontium lines appear continuous through both spectra, it is clear that the first spectrum is at least in part that of strontium.

As we have seen already, if we pass a spark in air between metallic poles we get the air lines as well as those due to the metal. We may use this comparison method to distinguish between the air lines and those of the metal. For let one set of poles be made of the metal in question, and take for the other set some metal with a simple known spectrum, platinum for example. Arrange the apparatus as described to observe the two spectra. The lines common to both are either air lines or are due to some common impurity of the two metals; the other lines in the one spectrum are those of platinum, in the second they arise from the metal in question.

After practice it is quite easy to recognise the distinctive lines of many substances without actual comparison of their spectra with that of a standard.

Experiment. - Compare the spectra of the sparks passing between platinum poles and poles of the given metal. Note the wave-lengths of the principal lines in the spark spectrum of the given metal.