110 
Browning^ on the Application of 
whichever position it might be employed, the angle made by 
the incident and emergent rays should be the same. 
I succeeded in making such a prism. Figs. 3 and 4 repre- 
sents this prism used in two diflPerent positions and fulfilling 
the required conditions_, i and i' being the same angle as i 
and i\ 
For most absorption-bands, particularly if faint, the prism 
will be used in the first position, in which it gives the least 
dispersion (Fig. 3) ; while whenever greater dispersion is re- 
Fig. 3. 
quired, so as to separate some particular lines more widely, 
to show the spectra of the metals or Fraiinhofer^s lines in 
the solar spectrum, then the prism must be placed in the 
position shown in Fig. 4. 
Mr. Sorby has informed me that for liquids or transparent 
Fig. 4. 
objects nothing could work better than this contrivance, but 
it is, of course, not applicable to opaque objects. 
Conversing with our President and Mr. Slack, after the 
last meeting, I mentioned that I believed that some form of 
direct-vision prisms, applied in the body of the microscope, 
would prove the best arrangement to answer both purposes, 
and Mr. Slack expressed the same opinion. 
Fig. 5 represents such an arrangement made with a com- 
