poten ty Royal Microscopical Society. 297 
the incidence, at the base, the angle of refraction will be represented 
by the other angle opposite the other side. I may mention one 
more fundamental principle. Aberration is at a minimum only 
when a ray of light passes through a prism at a minimum devia- 
tion. 
Mr. Browning’s very admirable spectroscope, exhibited at the 
Royal Society, depends upon this principle for its profound and 
truly astounding powers of spectroscopic analysis. And I do not 
hesitate to pronounce the opinion that through this subtle instru- 
ment alone his name will be handed down to posterity. It is one 
of the happiest ideas of the practical embodiment of principle. 
A prism like a lens forms two foci, primary and secondary ; and 
for the purposes of the most subtle analy sis which has ever engaged 
the human intellect, it is evident the most subtle and perfect powers 
of differentiation and resolution were absolutely necessary. By a 
series of prisms automatically arranged so as in each case to obtain 
the finest definition by means of preserving a minimum aberration, 
very extraordinary results are now obtained. 
The Huyghenian eye-piece was invented on the principle of the 
angles of immergence and. emergence being nearly equal, as in the 
case of the prismatic minimum deviation ; and by good luck (most 
of the best inventions are lucky thoughts) the same principle ren- 
dered the eye-piece achromatic. 
Now the microscope is really an instrument formed of innu- 
merable prisms. The action of any and every lens can be traced 
on the prism principle.. 
