The giant objective prism of the McClean Telescope. This may have been the largest such prism made at the time. There is a second (unmounted) objective prism in the Museum that was sometimes used in tandem with this one. The angles of the prisms were 8 and 12 degrees. The first was made by Howard Grubb and the second by Zeiss, Jena.
McClean used an objective prism on the Astrographic telescope to discover the presence of oxygen in stars.
A star photograph taken with the objective prism in place over the objective lens of the telescope. The action of the prism is to spread out the light of each star into its spectrum. In the centre is the unstable supergiant emission-line star Eta Carinae.