This small Browning spectroscope in the collection belongs to the category of direct-vision spectroscopes, which are instruments of great simplicity and are easy to use. Their dispersive apparatus is a direct-vision prism which, as we saw in the description of the previous instrument, combines good dispersion and practically no mean deviation, thus allowing the observation of the spectrum in the same direction from which the light comes.
The exemplar we present is composed of a brass tube with a diameter of 1.5 cm and a length of 8.5 cm, on which we find the brand and the manufacturer's name; on the inside of this tube slides another tube with a diameter of 1.4 cm and a length of 6.7 cm.
The inner tube contains the direct-vision prism and has at one end a converging lens which acts as a collimator, and at the other end a hole where the eye is placed.
This spectroscope, like most small, direct-vision spectroscopes is without a telescope. It is focused by moving the inner tube so that the slot reaches the front focal plane of the collimating lens; the monochromatic images of the slot supplied by the prism form at infinity and then on the retina of the observer, whose eye is adjusted to infinity.
In the photograph it is shown together with another direct-vision spectroscope made by the same manufacturer which is described in the following item.
Battelli - Cardani (s. d.), Vol. II, p. 564
Fleury - Mathieu (1966), Vol. 5, p. 65
Persico (1932), p. 627
Perucca (1949), Vol. II, p. 321