lO 



SMITHSONIAN' MISCELLANEOUS COLLEGTIONS 



VOL. 8 1 



The use of Bouguer's formula is unsafe for banded absorptions, 

 except possibly for a very pure spectrum, and as an interpolation 

 formula. Langley ' long ago showed its inapplicability in a region 

 where quite different coefificients of absorption occur, and his logic is 

 even more applicable in the present case where these occur in close 

 juxtaposition, and in banded spectra where the resolving power is 

 comparatively poor. Safer substitutes for Bouguer's formula may 

 be employed. For instance, in estimating atmospheric precipitable 

 water the writer always uses an absorption curve calibrated as far 

 as possible in the laboratory. A curve approximately of the shai)c 



V 



c 



M 

 o 



^ AUsorbenl: "thickness 



iMCi. 7- 



indicatetl in figure 7 would be expected. \\'here lines of strong 

 absorption occur alternately with those of high transmission, the 

 curve of figure 7 does not tend to approach a zero value of / with 

 increasing absorbent, but to l)ecome horizontal for a finite value of /. 

 Assuming Bouguer's fornnila to hold we should have a straight line, 

 tangent to some portion of this curve. In view of the state of affairs 

 indicated in figure 7, we should hesitate to use Bouguer's formula 

 for computing the amounts of ozone, unless for data requiring very 

 little extrapolation from the amounts of ozone used in the laboratory 

 to determine the constant of the formula. It may be that these con- 

 siderations explain certain discrepancies l)etween Dr. DobstMi's results 



' Ann. Astroplij's. Observ. Smithsonian Tnst. 2, lO, 1908. 



