Abstract: Disclosed is a sightglass assembly of particular application for use with a laser flowmeter to measure flow velocities of high pressure fluid flowing through a tube. A short, thick-walled glass tube having spherically rounded ends is coaxially interposed in a fluid flow system such that a continuous cylindrical fluid flow passage is provided through the bore of the sightglass tube. The sightglass tube is longitudinally clamped between a pair of endpieces having fluid flow bores. Spherical receiving surfaces in the endpieces are mated with the spherical ends of the sightglass tube to seal the joints between the sightglass and the endpieces and to pre-stress the sightglass tube with radially inward stress components enabling it to withstand higher internal fluid pressures. A pair of flat viewports on opposite sides of the sightglass tube provide windows for passage of laser light through the fluid medium in a direction transverse to the direction of flow.