ELECTRIC RECORDING PERIMETER.

Abstract

Of the many perimeters which have been devised for recording the field of vision, the instrument devised by Dr. Malcolm M. McHardy, of London, has met with the most favor. It is an excellent instrument, and whatever its defects, they are to be found in all similar forms of apparatus in which the test object is moved by a flexible cord. The instrument here shown was devised with a view of correcting the faults which experience shows exist in the ordinary form of perimeter. Apart from the use of small electric lamps for the fixation spot and for the test object, it differs in important details of construction from the usual form of perimeter. The arc which ordinarily carries the movable test object is replaced by a steel tube, at the end of which are cog-wheels for rotating a metal arm carrying a small electric lamp. Motion is transmitted to