Comte did not make proper observation of the visual spectra of sparks and flames in the laboratory. An understanding of this could have given him an idea of the visual spectra of stars, hence an idea of their compositions.
An advantage we have over Comte is the invention of the spectroscope in the 19th century. Comte did not know that the elements in the sun absorb light at specific wavelengths which make it possible for us to study its composition.
