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Since writing the editorial, "On Viewing the Eclipse," in last month's Archives and recommending a filter having an absorption not less than that of a Neutral Density No. 4, it has been discovered that standard filters of this density are not as readily available for comparison as was stated. Moreover, according to a bulletin which Kodak is to put out, infrared transmittance of Wratten filters is high. Hence these will no longer be recommended for eclipse viewing. We have therefore undertaken (at the suggestion of Dr. Virgil Casten) to "calibrate" filters of known density with the visibility of a 60-watt incandescent, frosted electric light bulb. It so happens that a filter No. 4 corresponds to the threshold at which the printed letters of a commercial bulb are just perceptible. Any filter should therefore be safe which reduces the intensity sufficiently so that the printed code on a standard 60-watt frosted
Cogan DG. MORE ABOUT THE ECLIPSEFilter for Viewing. Arch Ophthalmol. 1963;70(1):138. doi:10.1001/archopht.1963.00960050139021