This instrument is used in astronomy for the indirect measurement of apparent stellar diameters by counting the interference fringes and measuring the corresponding wavelength. The instrument consists of two bronze sleeves. On the face of one of the two sleeves there are the slit-carrying slides while the other sleeve, which remains external to the eyepiece of the telescope, carries the position circle and the eyepiece. The instrument is also equipped with a printer position circle with numbers and strokes that can be printed on a strip of paper using an ink-soaked ribbon.