NOTES AND QUERIES. 47 

but the underside of the diatom being in air (a less refracting 
medium) a portion only of the light escaped, owing probably to 
minute undulations of the surface, the greater portion being totally 
reflected upwards. 
In recent discussions in America, the illumination with the vertical 
illuminator had been assumed to be ‘‘opaque” as a matter of course, 
and many microscopists had allowed the assumption to pass with- 
out a critical examination. He thought, however, that Mr. 
Stephenson had completely explained the illumination to be o¢ 
“opaque.” 
Mr. Powell said he did not by any means insist that the illumin- 
ation shown was strictly that known as “opaque”; but he might 
mention that with an opaque object like a diamond-beetle’s wing, 
the illumination by the vertical illuminator was strictly “opaque”— 
that is, the object was viewed by means of the light which it 
reflected. 
_ Mr. Crisp, on removing the eyepiece of the microscope in Mr. 
Powell’s exhibit, said the portions of the bright ring of light that 
could be seen, really represented the zone of aperture that was 
effective in the new objective deyond the equivalent of 180° in air; 
in fact, the central portion of the aperture exactly corresponding to 
180° in air (z.¢., 82°, twice the critical angle in the body of the front 
lens) was practically inoperative in the illumination, the whole of 
that portion of the pencil was transmitted by the base of the cover- 
glass—producing practically no visible effect on the diatom—and 
was thus lost to the eye. 
Mr. Stephenson asked if Mr. Powell had been able to resolve 
the diatom using the vertical illuminator with an objective of less. 
aperture than 82° measured in crown glass? If the object were 
opaque enough to be viewed by reflected light it should then be 
seen. In his experience, however, under these conditions little or 
nothing but the mere outline of the valve could be seen, and 
nothing whatever of the resolution as now exhibited by Mr. Powell. 
He could not regard it as a case of “opaque” illumination. The 
diatom adhering to the cover-glass allowed the light beyond the 
“critical” angle to enter, but the under side of the diatom totally 
reflected back a great portion of the light—acting, indeed, to a 
great extent as a total reflecting surface. As he had previously 
pointed out, this method of illumination was a practical demonstra- 
tion of the excess of aperture of wide-angled immersion objectives 
beyond the equivalent of 180° in air,—not only of the existence of 
such aperture, but of its utilisation in obtaining resolution of difficult 
objects. : 
Mr. Powell said that with an objective of less than 82° angle in 
glass the diatom was practically invisible, and, of course, the bright 
ring of totally reflected light was not then seen. This ring was 

