Optical systems

An optical system of unit magnification for relaying an optical image having two first biconvex rod lenses arranged end to end on a common optical axis in spaced relationship, and between said first rod lenses but not in contact with them a pair of second rod lenses also arranged end to end on the same axis, the adjacent ends of the second rod lenses being concave and defining a biconvex cavity adapted to contain a fluid, whereby a fluid lens is formed, and the other ends of said second rod lenses being convex, the system being substantially symmetrical about a median plane through the biconvex cavity and the modulus of the reciprocal of the radius of curvature of each surface of the fluid lens being less than or equal to the modulus of the difference between the reciprocal of the radius of curvature of each of the convex surfaces of the first rod lenses adjacent to the second rod lenses and the reciprocal of the radius of curvature of each of the convex surfaces of the second rod lenses adjacent to the first rod lenses.