The achromatic prism is a system of prisms having different refraction indices, which gives a deviation independent of wavelength; the simplest one is with two prisms and, once two pieces of glass having much different refraction indices have been chosen, to produce it, it is sufficient to determine the refractivity angles and impose the condition that the deviation shall be the same for two monochromatic components fairly far apart (for example Fraunhofer's line C in the red and F in the blue) and that has a fairly small (a few degrees) pre-established value for an intermediate component (the Fraunhofer line D in the yellow).
The device presented here consists of two prisms, one of crown glass and the other of flint glass (refractivity angle about half of the former), with a frame of burnished brass; they can be superimposed but are upside down with respect to each other; they are attached to a swivel joint at the top of an extendible staff on a round brass base. They can thus rotate above the staff and also around two axes at right angles to each other.