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Simple optical methods, particularly the use of a cylindrical lens, are described which facilitate the examination of electrocardiograms and the differentiation of atrial flutter from atrial tachycardia with block. Results of utilizing the methods indicate that what may seem to be an isoelectric line of the latter is sometimes but an optical illusion, especially at slow atrial rates. Continuous atrial electrical activity of flutter has been detected at atrial rates as slow as 88 per minute. Some of these findings are relevant to certain considerations on the mechanisms of atrial arrhythmias.