In the type of television receiver most commonly used at the present time, the picture is produced on the fluorescent screen of the cathode ray tube, which is viewed directly or through a suitably placed plane mirror. The 9- or 12-in. cathode ray tubes normally used give pictures of about 8 in. by 6 in. in dimensions, while a somewhat larger picture is obtained with a 15-in. tube. This, however, probably represents the practical limit of picture size in this type of domestic receiver, since larger cathode ray tubes are unwieldy and expensive items. At the same time, it is desirable that the television receiver of the future should produce a picture of about twice the above size in linear dimensions, and preferably on a flat screen. With this object in view, developments had begun several years ago of a reproducing system using a small cathode ray tube with a fluorescent screen only two or three inches in diameter, on which a bright clear image is produced, the light from this image being then projected through a suitable optical system to give an enlarged picture on a flat screen. While considerable progress has been made in cathode ray tube design, the picture brightness obtainable on the tube is insufficient to give satisfactory results when using an optical system of the conventional type. For this reason the Schmidt optical system, which was originally designed for use in astronomical telescopes, has been applied to the projection of television pictures. The great advantage of this system is that it has a large light-gathering property, and so enables the utmost use to be made of the picture-brightness on the cathode ray tube.