A lamp provides the source of light. The beam of light strikes the diffraction grating (spectroscope) and separates the light into its component wavelengths.
The grating is rotated so that only a specific wavelength of light reaches the exit slit. Then the light interacts with the sample.
Aim the spectroscope at the object. Rotate the spectroscope until you clearly see a bright line spectrum on the side of the tube. (If you see a continuous spectrum, keep on rotating until the bright line spectrum is visible
Often refractive index (R1) and specific (SG) reading alone cannot identify gemstones.
More often than not you will need a Spectroscope to make that final determination.