Some Inve: 
in: 
graphy fo 
deal of w 
Departmen 
ly concen 
on and po 
graphing 
filter ou 
ferentiat 
species. 
able to € 
other. | 
picture. : 
the cond: 
ent ligh 
discussi 
remissio 
leaves a. 
in 1947. 
meter an 
Spectral 
ments wa 
1953... .I 
ferent 1 
present 
siderati