16 
according to the diminution of the aperture intensely black 
bands or borders are surmounted at their intersections with 
brilliant lozenge-shaped spectra, which rotate slightly as the 
focus is changed. 
The same thing should happen in the microscopic appear- 
ance of minute intersecting hairs. But this is never seen, 
except the corrections be of a very high order. On the 
contrary, every fine hair appears obscured with a blurred 
edging, under inferior optical treatment. 
The rhomboidal spectra of the finest hairs seen crossing 
at a very acute angle of 10° are remarkable; the black 
shadow is prolonged as seen at a, fig. 9. The hairs observed 
were about ;>4,;th of an inch in diameter, tapering to a fine 
point, nearly invisible. 
Error C.—When there is a complex structure in adjacent 
strata, the images of the substrata interfere with those of the 
upper, and false appearances result. 
Experiment 15.—If two pieces of fine wire gauze superim- 
posed be placed under the microscope, with a low power of 
twenty-five diameters, a series of false spectra appear, accord- 
ing to the plane of focal vision. 
Example.—W efts being inclined about 5°, to see the inter- 
ference lines a low four-inch power should be used. Extra- 
ordinary spectra exhibit false black dots, perfectly well 
defined. The distant bars vanish and form dots in the 
front. Crescentic lights and shadows, round dots, and a 
variety of appearances, present themselves, all of which 
possibly explain many false micro-spectra (fig. 12). 
A careful arrangement of spider lines would doubtless 
give some interesting results, as the “blue band” of the 
web described by Sir D. Brewster. 
Error D.—Nothing is more striking than the change of 
the markings produced by intersecting rouleaux, according 
as they are opaque or transparent, and the aperture and 
aberration are changed. Thus, in the great standard test, on 
which much will doubtless yet be written, the Podura, the 
bodies! of whose “ spines” form waves, are themselves spectral ; 
under correct adjustment they appear provisionally as elon- 
gated narrow cylinders, exquisitely distinct. Then, if the 
light be made sufficiently oblique, and the concave mirror be 
reduced in aperture, these cylinders show a double set of 
black lines of interference, alternate portions fade away, and 
a new appearance is put in—a very interesting example of 
interference rays. The lines do not correspond to the black 
' The standard form still clung to by the old school of microscopists. 
