device which is to receive the radiation of the second ellipsoidal mirror can be moved easily relative to the object but there are some systems in which it is necessary to maintain a fixed spatial relationship between the object and the image in such a system the arrangement shown in fig2 would not be satisfactory the drawback referred can be overcome by providing a mirror arrangement of the type shown in fig3 of the drawings fig3 of the drawings illustrates a continuously variable magnification optical system comprising a first pair of ellipsoidal mirrors 30 and 31 and a second pair of ellipsoidal mirrors 32 33 the mirror 30 is mounted upon a pivot 34 which is attached to the chassis of the optical system and the mirror 32 has a pivot point 35 which is also attached to the system chassis the mirrors 31 and 33 have a common intermediate pivot point 36 which is capable of moving with respect to the chassis in fig3 and 3 a reference point 37 is the image focus for mirror 30 and the object focus point 31 reference point 38 is the object focus point for mirror 32 and the image focus point for mirror 33 in the arrangement shown in fig3 the two pairs.