Pogson scale[′päg·sən ‚skāl]
a relation between the magnitudes m \ and m 2 of two stars and their brightnesses E1 and E2:
m2 — m1 = —2.5 log E2/E1
The value of —2.5 for the coefficient was proposed by the English astronomer N. R. Pogson in 1859. It was chosen so that a variation in brightness by a factor of 100 would correspond to a difference of five magnitudes (which is the mean difference between the brightest and faintest stars visible to the naked eye); thus faint stars have greater magnitudes.
The Pogson scale for visual observations is one example of the Weber-Fechner psychophysical law. In this case, the strength of the stimulus on the pupil of the observer’s eye is determined by the brightness of a particular star, while the intensity of the sensation is a function of the star’s magnitude.