Robert Bunsen

Robert Bunsen (1811-1899) was a German chemist. In collaboration with Gustav Kirchhoff, he pioneered the development of emission spectroscopy, a technique they used to discover the elements caesium and rubidium. Bunsen's most famous legacy is probably his invention of the Bunsen burner, which remains a standard piece of laboratory equipment in many sciences.

Interestingly, Bunsen's collaborator Kirchhoff also wore a neckbeard. They are pictured together in the photo below, with Kirchhoff on the left:

Alexander Bain

Alexander Bain (1818-1903) was a Scottish philosopher in the empiricist tradition who made major contributions to philosophy of mind and who helped to establish the use of scientific methods in psychology. He is perhaps best known as the founder of the journal Mind, which was originally intended as an interdisciplinary journal of philosophy and psychology, and which remains one of the most highly respected philosophical journals.

Notice that in his later life, Bain wore an interesting variation on the neckbeard style by shaving down the middle.