ks Langley— Atmospheric Absorption. 173: 
We have only to assume that &, J, etc., are sufficiently near 
‘zero (so that K, L and all other rays affected with such coeffi- 
cients sensibly vanish before they reach the observer), to see- 
that the only quantities sensible to observation are those with 
relatively large coefficients as A, B, 0, D, ete., so that now 
Aa +Bb +Ce +Dd +ete.=M ; 
Aa’ + BO’ + Ce’ + Dd’ + ete. =N 
Aad + Bb? +Ce*+ Dd’ +etc. =O 
From these values M, N, O, etc., it is plain that we can never 
estimate the amount of the extinguished rays K, L, etc., since 
these do not enter into the observed values by any amount 
sensible at all. 
Now to the rays A, B, ©, D, etc., which remain, and to their 
coefficients, we may evidently assign any values consistent with 
the conditions, which shall make the. difference between — and 
N 
N - 
5 3 small as we please, for in the equations 
Aa +Bb +Ce +Dd +ete,  Aa’+Bo*'+Cc?+Dad'+ete._1, 
Aa’ + Ba? + Ce? + Dd? + etc. Aa’ + Bd’ + Ce? + Dd* + etc. 
the conditions actually are, in inferring it from the ordinary 
formula, we must now consider more narrowly how this tellu~ 
