A large aperture aplanatic lens not corrected for colour

A lens suitable for spectroscopic work with aplanatic corrections for all zones may have as large an aperture as f/1 or still greater, all the surfaces being strictly spherical. An actual lens made by Messrs Ross, Ltd, of 3 inches focal length and 3 inches aperture, possesses corrections comparable with those given by the theoretical investigation. With a slightly reduced aperture, correction for colour may be obtained without prejudice to the quality of the spherical corrections. The investigation shows that the production of suitable glass discs is the outstanding difficulty in the way of great increases in the relative apertures of telescope objectives.