References:

• http://www.astrosurf.com/luxorion/Sciences/kirchhoff-lines-sun.pdf 

• https://web.mit.edu/spectroscopy/history/history-classical.html

• https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/spectrum/spectra_more.html

• https://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html

• https://archive.org/details/discoveryoftheel002045mbp/page/626/mode/2up

• https://www.jstor.org/stable/27852648

• http://www2.lowell.edu/workshops/slipher/Slipher1913LowOB2_56S.pdf

• https://www.researchgate.net/publication/277664445_Nebulae

• https://www.youtube.com/watch?v=UlwsDKYoRvc

• https://agu.confex.com/agu/abscicon19/meetingapp.cgi/Paper/480507