128 THE ANATOMY OF SCIENCE 



seen how near the greater science of mechanics 

 is approaching to a similar state. Indeed, as 

 we survey all the simple phenomena comprised 

 within the science of physics (leaving the so- 

 called irreversible processes for our next chap- 

 ter) we find that it is only in certain phases of 

 the subject related to the theory of radiation 

 that any dissymmetry is assumed between past 

 and future. Hitherto it has seemed impossible 

 to picture the process of absorbing light as 

 strictly the reverse of the process of emitting 

 light. In the old theory we thought of emission 

 as a series of concentric waves emanating from 

 the emitting body, but we did not consider the 

 absorption of light as associated with a series of 

 concentric waves converging upon the absorb- 

 ing body. If we can now devise a theory in 

 which emission and absorption are quite sym- 

 metrical it will greatly simplify our concept of 

 physical time. 



When we consider once more the simple ap- 

 paratus of Figure 22, where C now represents 

 the center of a dark interference band, we see 

 that if we cover the two mirrors, and then un- 

 cover the mirror AA' for an instant, a corpuscle 

 of light may proceed from S to A' and from A' 

 to C ; but if we uncover both mirrors for the 



