Light, coming from the luminous source that we want to study, passes through the slit of the first telescope and, by the lens situated inside, focuses on the prism. The speed of propagation, through the glass that composes it, is different for each of the electromagnetic radiations that the beam of light is made of, so they refract at different angles. Consequently, they come out of the prism separated according to their different frequencies.
If now we observe the light emerging from the prism with the third telescope, we’ll see a set of coloured stripes (images of the slit), which makes the so called ‘spectrum’ of incidental radiation. In it, certain lines of colour can appear against a black background (emission spectrum) or black lines against a coloured background (absortion spectrum), whose position and width can be measured by using a graduated scale. This scale is incised in the second telescope, but is cast on the prism and appears superimposed on the spectrum that we are studying.