Abstract: An image tube for electrical measurement of optical images comprises a vacuum vessel having a photocathode at one end and a mesh intermediate its ends. The images are produced with high resolution when the distance from said photocathode to said mesh (L.sub.1) and the total extension (L.sub.2) along a central axis of the tube in the path of movement of the electrons are selected by means of the following mathematical relations: ##EQU1## the symbol n denoting an integer indicating the number of revolutions rotated by the electrons along their path of movement.

Abstract: The invention relates to a coil for producing in a cylindrical space a magnetic field being homogeneous in a direction parallel to the axis of said space, which coil comprises a cylindrical winding concentric with the cylindrical space and having at least one pair of mutually identical windings placed symmetrically with respect to the medial plane of the cylindrical winding perpendicular thereto, and extending in planes perpendicular to the longitudinal axis of the cylindrical winding. The pair of mutually identical windings may be situated at the opposite ends of the cylindrical winding and may have a closeness of winding higher than that of the cylindrical winding.

Abstract: The invention relates to a method of obtaining stigmatic imaging in spectral analysis using a concave grating in offplane mounts. The invention comprises placing on one hand the object, which may be an entrance slit, and on the other the focal surface in such a manner with respect to the concave grating that certain conditions specified in disclosed formulae and relating to an othogonal three dimensional coordinate system the origo of which lies in the middle of the grating surface, the X-axis of which runs along the normal to the grating surface, and the Z-axis of which is parallel with the grooves of the grating, are satisfied at a value for the angle between the XY-plane and a line from a selected point on the object to origo that differs from zero.