436 HAROLD F. BLUM 



and the process is independent of the intensity. This is general^ 

 expressed by the relationship: 



intensity X time — a constant (11) 



which is called the reciprocity law, or sometimes the Bunsen-Roscoe 

 law. It is obvious that such a relationship must hold for the primary 

 act (equation 1) but it should be equally obvious that it may or may 

 not hold for the over-all process, because, if any intermediate step is 

 dependent upon the intensity, the end response must also depend upon 

 the intensity, e.g., a reversible intermediate reaction may depend 

 upon the rate at which some substance is formed in the primary reac- 

 tion, and hence upon the intensity. As a matter of fact, reciprocity 

 is displayed by many photobiological processes over a wide range of 

 intensities, and it is probable that numerous instances of failure that 

 are described are apparent rather than real; but failures are never- 

 theless to be anticipated. 



One cause of apparent failure of reciprocity is that the time re- 

 . quired for a given biological end point to be reached includes the time 

 required for processes set off by photochemical reactions, but which 

 are not themselves dependent upon light. For example, photosensi- 

 tized red blood cells hemolyze after sufficient exposure to light. If 

 the exposure is terminated at the proper time, before the cells have 

 begun to disintegrate, they will, nevertheless, hemolyze in the dark 

 later on. Obviously, if one continues the exposure until, say, 50% 

 hemolysis, one includes, in calculating the dose, a period during which 

 the process is more or less independent of the light impinging upon 

 the system. In this case departure from reciprocity is observed. 

 On the other hand, if one exposes the cells to a small dose, such as will 

 cause hemolysis to occur at some later time, say 24 hours after ex- 

 posure, the magnitude of the dose can be accurately calculated, and 

 when this is done reciprocity is found. The determinations in the 

 latter case require that a series of hke samples of cells be exposed to 

 various doses, at various intensities, and then left for 24 hours, at 

 the end of which time the minimum dosage that has produced hemoly- 

 sis is determined {2, Ch. 4). 



We have here an example in which the end point is a specific 

 biological response set off by a dose of radiation delivered at some 

 earlier time. This is a common type of criterion. Other examples 

 are the killing of bacteria by ultraviolet radiation {18,22) and the 

 induction of tumors by repeated doses of ultraviolet radiation, the 



