Spectroscopy is a broad science. It has a lot of aspects from visible light to the spectra of stars. Therefore, there is not just one instance in which spectroscopy was discovered. In fact, many people contributed to the discovery of different subcategories of spectroscopy. Below are some of the major discoveries within the different subcategories. 

• Isaac Newton in 1666: Newton repeats the prism experiments of Descartes, Grimaldi, Hooke, and others to explore deeper into the order and composition of the colored spectrum. He then found that white light is of composite nature. However, his experiments failed to unveil the absorption lines within colors due to the limitations of his equipment. 

• William Herschel in 1800: Herschel investigated the heating power of the solar spectrum by allowing a prism to place the various parts of the spectrum on a thermometer. Herschel discovered that the hottest part of the spectrum was 1.5 inches beyond the red portion. More related to us though, he discovered that the green and yellow portions hold the maximum visible brightness of sunlight. Really, the heat and brightness that he was experimenting with is how we interact with sunlight daily.

• William Hyde Wollaston in 1802: Wollaston published a paper regarding his work in producing the solar spectrum. During which, he noticed dark lines between the distinctive colors. He believed that these were simply just gaps that separated the colors for some reason.  

• Joseph Fraunhofer in the early 1800s: Fraunhofer started work in a glass factory responsible for the production of military and surveying instruments. His first success was the development of optical crown and flint glass free from bubbles. This glass he would then discover was a very useful optical instrument. Wanting to perfect optical instruments, Fraunhofer quickly became interested in the visible spectrum and therefore spectroscopy. As he repeated experiments close to those of Isaac Newton, he instead discovered strong and weak vertical lines within the spectrum. These are commonly referred to as Fraunhofer lines, as I mentioned before. He was convinced that the lines were not color boundaries but something else. His discovery led to more experiments of his own and of others.  

• Thomas Young in the mid 1800s: Young was investigating the relationship between interference and diffraction of light. Using a 500 groove per inch system, he successfully assigned value to the color spectrum. His measurements ranged from 675 nanometers in the far red to 424 nanometers at the violet limit. Since then, we have used this to determine when we see color. 

• Giovanni Battista Donati in 1860: Donati published a description of the spectra of around 15 stars. 

• Lewis Rutherford in 1863: Rutherford was the first to try the classification of spectra of stars into multiple groups.