ULTRAVIOLET LIGHT — GERSHENFELD 211 



to 4000 Angstrom units) . In another more convenient breakdown, the 

 ultraviolet spectrum is divided into four parts, the divisions or classes 

 being based upon the use of these specific radiations or so divided 

 based upon their principal effects. From 1000 to 2000 Angstrom 

 units are radiations which form a toxic gas, ozone, the so-called ozone- 

 forming ultraviolet radiations. In the range from 2000 to 2950 

 Angstrom units are to be found radiations which are effective in 

 destroying germs. In the range from 2800 to 3300 Angstrom units 

 are to be found the "biologically effective" radiations, those produc- 

 ing sunburn; and because they also are useful in activation of vitamin 

 D, they sometimes are called the antirachitic ultraviolet radiations. 

 In the range from 3300 to 4000 Angstrom units or the region closest 

 to the visible spectrum are to be found radiations useful for their 

 fluorescent display nnd of value in souie types of photography. These 

 divisions are not vei-y sharjily defined; they frequently merge and 

 often actually overlap considerably. However it is important to note 

 that the cliemical a?id physiological actions of these rays depend 

 upon tlie length of the waves making up these different radiations. 



TRANSMISSION OF ULTRAVIOLET LIGHT 



Before considering the value of ultraviolet light as a sanitary tool, 

 there are other considerations which should be directed to your 

 attention. All rays of the sun pass through the atmosphere before 

 they reach the earth. The ordinary atmosphere will allow the pas- 

 sage of infrared rays and the rays of the visible spectrum. Most 

 ultraviolet rays from the sun, however, fail to reach the earth. Ac- 

 tually the air acts as a differential filtering medium holding back 

 many rays. In most places radiations shorter than 2900 Angstrom 

 units never penetrate to the ground; in fact the shortest to reach 

 us is usually from 2950 to 3100 Angstrom units in length. Less 

 than 0.1 percent of the total radiations from the sun is in the form 

 of ultraviolet rays and almost all of the latter are longer than 2900 

 Angstrom units. Furthermore only the long ultraviolet rays will 

 pass through ordinary window glass. Wliile the latter is trans- 

 parent to the visible light rays, it is opaque to light of wave lengths 

 shorter than 3100 Angstrom units. You will note from these observa- 

 tions that sunlight reaching us therefore is most frequently devoid 

 of an active germicidal effect and that is especially the case of sun- 

 light that has passed through an ordinary pane of window glass. 

 On the other hand, a special type of commercial glass, and in par- 

 ticular quartz, allows the passage of ultraviolet light possessing wave 

 lengths shorter than 2900 Angstrom units. This is of paramount 

 importance in sanatoria and nurseries, where it becomes necessary 

 to use glass of special composition instead of the ordinary window- 



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