A spectroscope is a device which measures the spectrum of light. Early spectroscopes were simply a prism with graduations marking wavelengths. Modern spectroscopes generally use a diffraction grating, a movable slit and some kind of photodetector, all automated and controlled by a computer.
The spectroscope was invented by Gustav Robert Georg Kirchhoff and Robert Wilhelm Bunsen.
See also spectroscopy, spectrograph, spectrometer.