  This paper examines the relationship between spectra of stars of same spectral type with extremely low reddenings.   According to the standard theory, the relationship between the spectrum of stars with same spectral type and small, but different reddenings should be different in the optical and in the UV.   This difference is not observed: the ratio of the spectra of two stars in directions where the reddening is large enough to be detected and low enough not to give a noticeable 2200Ang. bump is an exponential of 1/lambda from the near-infrared to the far-UV.   This result is in conformity with the ideas introduced in preceding papers: the exponential optical extinction extends to the UV, and the spectrum of stars with enough reddening is contaminated by light scattered at close angular distance from the stars.   An application will be the determination of the spectrum of a non-reddened star from the spectrum of a star of same spectral type with little reddening. 
The standard theory of extinction and the spectrum of stars with very   little reddening