The following is from "Realm of the Universe," by George Abell, (c) 1976, p. 219, my old college text...

Magnitudes, whether apparent or absolute, that are based on stellar brightness as they are observed with the human eye are called visual magnitudes or absolute visual magnitudes. Today photographic plates or photomultipliers can be used in conjunction with many kinds of color filters to produce a great variety of different magnitude systems–for example, red magnitudes, ultraviolet magnitudes, infrared magnitudes, and so on. A certain few spectral bands have become more or less standard, however, and are now widely used to define magnitudes.

Since about 1960 the most commonly used magnitudes have been U (ultraviolet), B (blue), and V (visual). The U and B magnitudes are obtained from measures of the flux from stars through certain standardized ultraviolet and blue filters with a common type of photomultiplier. The visual magnitude (V) is measured with the same photocell through a filter that approximates the response of he human eye. The difference between any two of these magnitudes, say between blue and visual magnitudes (B-V), is called a color index. Since the inverse square law of light applies equally to all wavelengths, the color index of a star would not change if the star's distance were changed.

So, Chuck wins the memory prize on the subject. ---- Jim Cobb james@cobb.name