4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



its incident intensity value. These may be found from the same curve 

 by reference to the ordinates at the left within the frame. 



Thus at the limit of the visible in the red we find that some 30-cm 

 water path is required to reduce the intensity of light to one-half its 

 original value, whereas at i^ix only .03 cm or .3 mm will produce the 

 same result. As water cells are frequently used of i-cm thickness it is 

 convenient to indicate the wave-length range over which such a cell 

 will yield appreciable transmission. The values of transmission for 

 I -cm path are indicated at the right of the upper section. These 

 enable one to immediately estimate the wave-length range for the 

 cut-ofT from such a cell. 



In order to compare the absorption characteristics which are fa- 

 miliar to radiologists in the X-ray range with those exhibited in the 

 visible range the X-ray values have been indicated by the dotted curve 

 b, the wave lengths being found by reading Angstroms instead of /x at 

 the bottom of the graph. It is interesting to note the relatively smooth 

 transition from low to high transmission in the X-ray region com- 

 pared with the highly selective characteristics exhibited in the infra- 

 red, visible, and near ultra-violet. 



As the presence of ozone in the atmosphere plays an important role 

 in limiting the light which reaches the earth, the transmission char- 

 acteristics of ozone gas under standard conditions have been indicated 

 by curve c. The transmission values at the right now apply to a cell 

 of I -cm thickness of ozone gas under standard conditions. Since, 

 however, the whole absorption in the atmosphere is equivalent to about 

 3 mm, in order to estimate the absorption of atmospheric ozone it is 

 necessary to shift the transmission scale bodily upward by one-half 

 of one of the large spaces indicated. We thus find the transition from 

 90 per cent to i per cent occurring in a very narrow region from 

 3,200 A to 2,950 A respectively. 



With these curves in mind we may now profitably turn to the matter 

 of sources of radiation with which one has commonly to deal. For the 

 sake of comparison we have assumed that lamps will be chosen of such 

 a size and used at such a distance that a comparable amount of maxi- 

 mum energy is received. The curve at the left shows the relative 

 emission per unit wave length of radiation at each wave length for a 

 soHd body at the absolute temperature of 1,000° K. Here we find 

 most of the energy occurring for wave lengths longer than 1.4.(1, or, in 

 other words, in a region where practically all the energy will be ab- 

 sorbed in an extremely thin layer of water. The customary dull red 

 therapeutic lamp has characteristics not greatly different from this 



