Diagnosis of Diplopia.

SIRS,-The article by E. C. Cowan (Brit. J. Ophthal., 1960, 44, 59), describing a simple apparatus for the diagnosis and recording of diplopia, prompts me to make a few observations. His instrument uses a method not unlike that which I described some years ago (Brit. J. Ophthal., 1949, 33, 54) and which is still available in an improved form called the "Lees Screen" from Theodore Hamblin Ltd. Mr. Cowan appears to be unaware of this screen as he does not mention it in his article. My early experiments led me to conclude that the use ofa small screen, as in Mr. Cowan's apparatus, because of the short working distance involved, leads to errors from the eyes converging secondarily to the accommodation which is brought into play. The need to invert the screen by such a method as Mr. Cowan's was recognized by me, and was obviated on my screen by a double mirror and by screens on which the tangent scale is invisible until illuminated, so enabling the testing of each eye without moving the screens in any way. I came to the conclusion that portability, although occasionally convenient, was better sacrificed for the benefits which a larger and more refined, though static, instrument conferred. Yours truly, V. T. LEES. ROYAL ALBERT EDWARD INFIRMARY, WIGAN, LANCS. February 24, 1960.