The following text is from Concepts of Physics by Dr. H.C.Verma, from the chapter "Dispersion and Spectra", page 434, topic "Dispersive Power":
The dispersive power of a material is defined as the ratio of angular dispersion to the average deviation when a light beam is transmitted through a thin prism placed in a position so that the mean ray (ray having the mean wavelength) passes symmetrically through it.
After the definition of dispersive power, the author has derived the following expression based on the assumptions - the angle of prism is small and so the angle of deviation is small:
This equation itself may be taken as the definition of dispersive power.
In the above equation, $\omega$ is the dispersive power, $\mu_v,\mu_r$, and $\mu_y$ are the refractive indices of violet, red and yellow components of light respectively.
The equation for dispersive power was derived for a specific case - a thin prism (a prism with a small refracting angle). Then how could it be "taken as the definition of dispersive power"? This statement from the book seems to imply that it must also be applicable for dispersing elements other than "thin" prisms. Is the equation really valid for other dispersing elements like a prism of large refracting angle, or a glass sphere, or a grating?
Note: I haven't included the complete derivation of the formula as I thought it will increase the size of the post tremendously. However, I hope I have explained the main point clearly. If not, kindly ask your queries in the comments.
The Wikipedia article on Dispersion doesn't offer any explanation regarding dispersive power.
My search results on dispersing power didn't fetch any thing from reliable sources. So I decided to ask it here.