"The collimators and observing telescopes have apertures of 50mm., and focus by rack and pinion. Two 60 degree prisms of heavy flint glass and of 70mm. side are furnished with the instrument. The mirror may be used in different positions, to permit the use of one or two prisms. A small right angle prism, easily adjustable in every direction, is placed near the slit of the first collimator and throws the light into the slit of the second collimator, whence it is reflected by another right angle prism into the Echelon and to the observing telescope."