Some additional early references to erecting prisms of the type used in the Hastings Brashear binocular have been found as well.
Based on a comment from Larry Gubas that Zeiss used an Abbe-Koenig type of erecting prism in the pre-1900 Dosen-Fernrohr, I looked around and was able to find a cutaway view of that instrument in a 1921 English translation of the original German book by Gleichen:
The Zeiss Dosen - Fernrohr (Revolver Telescope) is shown in two models, Klienes and Grossen (small and large), in Zeiss 1899 and 1902 catalogs. It is the first use of the Abbe type roof erecting prism by Zeiss that I am aware of but I am not sure when this Zeiss telescope was first introduced.
The cutaway view in Gleichen shows a prism of the same construction as the one described in the 1897 Hastings US Patent not the modified type of construction patented by Hensoldt in 1905.
If anyone has an earlier reference to the Dosen - Fernrohr please let us know. The question still remains as to when Zeiss first used or described that type of prism in Germany.
Another interesting item is a picture in the 1906 Catalog of the John Brashear Company:
This shows some of Brashear's optical component fabrication capabilities at the time including what appears to be two Hastings type erecting prisms. The Brashear company supplied complete instruments as well as a great variety of optical components made to order. By 1906 he had probably made a lot of Type I Porro prisms for the Warner and Swasey binoculars, thus the "ring of Porro prisms" shown in the catalog display.
The true origins of the roof prism are unknown to me, in spite of some dedicated sleuthing. Probably the first roof prisms were made circa the 1880s, either improved or invented by Daubresse. Does anyone have more information? --Peter
From: Stephen Sambrook <scsambrook@___co.uk>
Subject: Combined objectives and prisms
I wonder if anyone knows of any early prism binoculars which formed their objective lenses by combining a crown convex lens with a with a concave-ground flint-prism face to give an achromatic combination ?
My ignorance of technical optics is probably unsurpassed, but I wonder if such an idea used in conjunction with a Sprenger or Leman prism and a cemented field lens (as found in the Barr & Stroud porro-2 glasses) might not have had potential for producing a binocular with the absolute minimum of air-to-glass surfaces and considerably enhanced stereopsis.