The telescope resolves light from very far away. The spectroscope analyses and diagnoses it. It is through spectroscopy that we know what the stars are made of. The spectroscope shows us that the universe is expanding and the galaxies receding; that time had a beginning, and when; that other stars are like the sun in having planets where life might evolve.
In 1835, Auguste Comte, the French philosopher and founder of sociology, said of the stars:
"We shall never be able to study, by any method, their chemical composition or their mineralogical structure . . . Our positive knowledge of stars is necessarily limited to their geometric and mechanical phenomena."
Even as he wrote, the Fraunhofer lines had been discovered: those exquisitely fine barcodes precisely positioned across the spectrum; those telltale fingerprints of the elements. The spectroscopic barcodes enable us to do a chemical analysis of a distant star when, paradoxically (because it is so much closer), we cannot do the same for the moon — its light is all reflected sunlight and its barcodes those of the sun. The Hubble red shift, majestic signature of the expanding universe and the hot birth of time, is calibrated by the same Fraunhofer barcodes. Rhythmic recedings and approachings by stars, which betray the presence of planets, are detected by the spectroscope as oscillating red and blue shifts. The spectroscopic discovery that other stars have planets makes it much more likely that there is life elsewhere in the universe.
For me, the spectroscope has a poetic significance. Romantic poets saw the rainbow as a symbol of pure beauty, which could only be spoiled by scientific understanding. This thought famously prompted Keats in 1817 to toast "Newton's health and confusion to mathematics", and in 1820 inspired his well known lines:
"Philosophy will clip an Angel's wings,
Conquer all mysteries by rule and line,
Empty the haunted air, and gnomed mine —
Unweave a rainbow . . ."
Humanity's eyes have now been widened to see that the rainbow of visible light is only an infinitesimal slice of the full electromagnetic spectrum. Spectroscopy is unweaving the rainbow on a grand scale. If Keats had known what Newton's unweaving would lead to — the expansion of our human vision, inspired by the expanding universe — he could not have drunk that toast.