NO. 8 RADIATION AND BIOLOGICAL DATA BRACKETT 5 



curve. For that reason it must be regarded for the most part simply 

 as a surface heater. 



The next curve indicates the emission of a soHd body at an absolute 

 temperature of 3,000° K, where it is now seen that its maximum 

 energy lies in a region which would be relatively well transmitted by 

 water. Such a radiation might well be expected to penetrate some- 

 what into the living matter. It does, however, contain a considerable 

 proportion of energy which will be absorbed in a thin layer, that is for 

 wave lengths longer than 1.4/*. If one wishes radiation that is as 

 nearly free as possible of this surface-absorbed energy, a light of this 

 temperature should be used with a water filter. A modified curve is 

 indicated terminating at approximately i.4fx, which shows the type of 

 radiation which one would receive from an ordinary high-temperature 

 lamp such as the customary Tungsten light when equipped with a 

 water cell of i-cm thickness. Again, on approximately the same scale 

 the relative distribution of solar energy is shown as it would be with- 

 out atmospheric absorption. Owing to atmospheric ozone no ap- 

 preciable ultra-violet reaches us from the sun beyond 2,950 A. As the 

 amount of ozone fluctuates this limit varies considerably. Further- 

 more, large amounts of energy are absorbed in the infra-red by at- 

 mospheric molecules, particularly water vapor. This, again, is subject 

 to extreme variations, depending upon the location, time of day, and 

 amount of humidity. In the solar curve we see that the chief energy 

 lies in the visible region, whereas our high-temperature lamp, even 

 with a water filter, has the larger proportion of its energy in the near 

 infra-red. Since for therapeutic purposes the mercury arc is very 

 widely used, its energy distribution has also been shown. As its light 

 is radiated chiefly in a large number of restricted regions of prac- 

 tically monochromatic light, it can best be shown simply by vertical 

 lines. The height of these lines is proportional to the intensity. Since, 

 however, they difter widely for different conditions of excitation, they 

 must be regarded at best as only a rough basis for estimation. 



In addition to the blue and ultra-violet lines with which we are 

 chiefly concerned, this arc shows not only strong yellow and green 

 lines, but in most cases a line at 1.014/^ of an intensity which exceeds 

 any other line. This great line in the near infra-red occurs in a region 

 where it is readily transmitted by water and, as we shall see in a 

 moment, to a great extent by flesh. This wave length of radiation is 

 readily transmitted by the aqueous humor of the eye and will be 

 chiefly absorbed in the retina. While undoubtedly ultra-violet effects 

 would be noticed long before any danger would be incurred from this 

 radiation in the case of a quartz mercury arc, on the other hand in the 



