"Iceland spar" Quotes from Famous Books
 ... as yet contrived and used for optical superimposition is a "double-image prism" of Iceland spar (see Fig., p. 228), formerly procured for me by the late Mr. Tisley, optician, Brompton Road. They have a clear aperture of a square, half an inch in the side, and when held at right angles to the line of sight will separate the ordinary and extraordinary images to the amount of two inches, ...
— Inquiries into Human Faculty and Its Development • Francis Galton |
