cit. "Thus the magnitude of the central spot is
diminished, and the brightness of the rings increased, by covering
the central parts of the object-glass."

[3] _"Man kann daraus schliessen, was moglicher Weise durch
Mikroskope noch zu sehen ist. Ein mikroskopischer Gegenstand z. B,
dessen Durchmesser = ([lambda]) ist, und der aus zwei Theilen
besteht, kann nicht mehr als aus zwei Theilen bestehend erkannt
werden. Dieses zeigt uns eine Grenze des Sehvermogens durch
Mikroskope"_ (_Gilbert's Ann._ 74, 337). Lord Rayleigh has recorded
that he was himself convinced by Fraunhofer's reasoning at a date
antecedent to the writings of Helmholtz and Abbe.

[4] The last sentence is repeated from the writer's article "Wave
Theory" in the 9th edition of this work, but A. A. Michelson's
ingenious echelon grating constitutes a realization in an unexpected
manner of what was thought to be impracticable.--[R.]

[5] Compare also F. F. Lippich, _Pogg. Ann._ cxxxix. p. 465, 1870;
Rayleigh, _Nature_ (October 2, 1873).

[6] The power of a grating to construct light of nearly definite
wave-length is well illustrated by Young's comparison with the
production of a musical note by reflection of a sudden sound from a
row of palings. The objection raised by Herschel (_Light_, S 703) to
this comparison depends on a misconception.

[7] It must not be supposed that errors of this order of magnitude
are unobjectionable in all cases. The position of the middle of the
bright band representative of a mathematical line can be fixed with a
spider-line micrometer within a small fraction of the width of the
band, just as the accuracy of astronomical observations far
transcends the separating power of the instrument.

[8] "In the same way we may conclude that in flat gratings any
departure from a straight line has the effect of causing the dust in
the slit and the spectrum to have different foci--a fact sometimes
observed." (Rowland, "On Concave Gratings for Optical Purposes,"
_Phil. Mag._, September 1883).

[9] On account of inequalities in the atmosphere giving a variable
refraction, the light from a star would be irregularly distributed
over a screen. The experiment is easily made on a laboratory scale,
with a small source of light, the rays from which, in their course
towards a rather distant screen, are disturbed by the