800 Janssen Solar Photograph and Optical Studies. 
the thousandth exposure may fall in the brief instant of defi- 
nition the observer patiently watches for, and then the results 
nomenon, or (what might conceivably be the case) some unde- 
tected causes of minute instrumental error. The difficulty of 
presence of residual phenomena, which, however minute, were 
i ty of distinguishing by @ 
single plate, the exact limits between the effects of solar and 
tremities of filaments; extremities whi aggregated, 
fs sg and which lifted higher than their fellows cause the 
* Comptes Rendus, Sept. 6, 1875, p. 438. 
