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An Instrument for Measuring the Distance betiveen the Centres 

 of Rotation of the Two Eyes. 

 By H. S. Rtland and B. T. Lang, F.B.C.S. 



(Communicated by George J. Burch, M.A., D.Sc, F.R.S. Received January 9, — 

 Read February 29, 1912.) 



A knowledge of the distance between the two eyes being required in 

 dealing with the problems of binocular vision, the instruments described in 

 this paper may be of interest to physiologists. All those methods which 

 involve measurement of the distance between the pupils (or other external 

 parts of the eyes) are liable to errors, for the following reasons : — 



1. It is difficult to eliminate parallax between the scale or index of the 

 measuring instrument and the eyes. 



2. The distance between the pupils is affected by the convergence of the 

 optical axes, and still more by the abnormal direction of either. 



3. It is difficult to ensure the eyes being kept still during the process of 

 measuring. 



By the method herein described, advantage is taken of the mobility of 

 the eyes, which is the cause of this very difficulty, to measure the one 

 distance that cannot vary, namely, that between the optical centres of 

 rotation. So far as we have been able to find, this has not been done 

 before. We have made two instruments, each involving the same general 

 principles in a different form. 



The first form (fig. 1) consists of a suitable base A, shaped at one end to 

 fit the forehead, and carrying at the other end a vulcanite plate B, upon 

 which slides a vertical index C, against a scale SS. In the base itself there 

 is a vertical index which may be placed at either D or E. When in use 



Fig. 1. 



