Fig. 193.
Fig. 193a.
Figs. 193 and 193a represent prisms of the kind arranged to use in two different positions, i and i′ being the same angle as I and I′.
For most absorption-bands, particularly if faint, the prism should be used in the first position, in which it gives the least dispersion; when greater dispersion is required, so as to separate some particular lines more widely, or to show the spectra of the metals, or Fräunhofer’s lines in the solar spectrum, then the prism must be used as in [Fig. 193]a. This answers well for liquids or transparent objects, but it is, of course, not applicable to opaque objects.
To combine both purposes, some form of direct vision-prisms that maybe applied to the body of the microscope is required. [Fig. 194] represents an arrangement of direct vision-prisms, invented by Herschel. The line R R′ shows the path of a ray of light through the prisms, where it would be seen that the emergent ray R′ is parallel and coincident with the incident ray R.
Fig. 194.
Fig. 194a.