Within these specified limits, I can find you a star of any weight or of any colour or of any size you like. But this does not mean that you may specify the weight _and_ colour _and_ size of the star you want, and that I will undertake to find it for you; if the weight is right the colour may be wrong, and so on. For instance, if you ask for a red star I can find you a very heavy one or a very light one, but it is no good your asking for one of intermediate weight. So far as we know, red stars of intermediate weight simply do not exist. The same is true as regards size—there are no red stars of intermediate size. Hertzsprung noticed in 1905 that the red stars could be divided sharply into two distinct classes characterised by large and small size—he called them Giants and Dwarfs. Russell, studying the question further in 1913, confirmed Hertzsprung’s earlier conclusions, and shewed that the giant-dwarf division extended to stars of other colours than red. Imagine that we have a series of coloured ladders, one for each colour of star—red, orange, etc. Take all the red stars and stand them (in imagination) on the different rungs of the red ladder. Do not merely place them on at random; arrange them in order of their luminosities, placing those of highest luminosity upper-most. Further let several stars stand on the same rung if their luminosities are about equal. To make the arrangement definite, let each rung of the ladder represent 5 times higher luminosity than the rung immediately below it, so that each rung has a definite luminosity associated with it[25]. [25] For purely practical reasons the height is not taken proportional to the luminosity but to its logarithm; without some such device as this it would be impossible to represent the range of more than 1,000,000 to 1 in the observed luminosities of red stars. With this agreement we are now ready to proceed. We take our red stars and place each on the appropriate rung of the red ladder, and so on for each other colour. The result is shewn diagrammatically in fig. 21, the different stars being represented by crosses. The red stars will be found to lie as on the right of the diagram, Hertzsprung’s division into giants and dwarfs being very clearly marked. The orange stars lie as on the next ladder to the left; as Russell found, the division again appears, but is less marked. THE RUSSELL DIAGRAM. Let us make ladder diagrams of this kind for each colour of star, and put them side by side in their proper order, so as to represent stars of all possible colours. We obtain a diagram of the kind shewn in fig. 22. This type of diagram was introduced by Russell in 1913, and is now generally known as a Russell diagram.