Abstract
Withrow 1 has described a series of dyed cellophane light filters for securing several sharp cut-offs in the ultra-violet region, and states that 2 sheets of cellophane soaked in sodium benzoate will absorb all radiations shorter than 2890 A.U., 2 sheets soaked in potassium acid phthalate absorbing all radiations shorter than 3130 A.U., and 3 sheets soaked in potassium acid phthalate absorbing all radiations shorter than 3340 A.U. In a later publication, however, the originator of the filters 2 states that the sodium benzoate filter excludes all radiations shorter than 2970 A.U.
To the investigator working with ultra-violet light, this discrepancy is important, since it means that 2 biologically active mercury lines, at 2894 and 2967 A.U., are either included or excluded in an irradiation, according to the cut-off of the filter. The author sent some new and used light filters that he had made up according to Withrow's directions to the DuPont Experimental Station at Buffalo, New York, and through their kindness was able to procure some very accurate transmission curves of both the new and used filters. It was found that the cut-off for the sodium benzoate filter was at 2890 A.U., but that there was an upward shifting of transmission amounting to 150 A.U. upon use, thus excluding 2 biologically active mercury lines. Summarizing the data obtained from the transmission curves of the filters: 1. Two thicknesses of a new sodium-benzoate soaked filter transmit the 2894 mercury line and exclude all shorter wave lengths; after more than 5 days of use this filter transmits only to 3024 A.U., thus excluding the extremely active 2894 and 2967 A.U. mercury lines. 2. Two thicknesses of a new potassium-acid-phthalate soaked filter transmit the 3132 A.U. mercury line and exclude all shorter radiations; there is no change in transmission in the filter in 5 days. 3.
Get full access to this article
View all access options for this article.