be put
equipollent to an infinitely little Ring; the Squares of the Diameters
of the Rings will be in the progression 0, 1, 2, 3, 4, &c. I measured
also the Diameters of the dark Circles between these luminous ones, and
found their Squares in the progression of the numbers 1/2, 1-1/2, 2-1/2,
3-1/2, &c. the Diameters of the first four at the distance of six Feet
from the Speculum, being 1-3/16, 2-1/16, 2-2/3, 3-3/20 Inches. If the
distance of the Chart from the Speculum was increased or diminished, the
Diameters of the Circles were increased or diminished proportionally.

_Obs._ 4. By the analogy between these Rings and those described in the
Observations of the first Part of this Book, I suspected that there
were many more of them which spread into one another, and by interfering
mix'd their Colours, and diluted one another so that they could not be
seen apart. I viewed them therefore through a Prism, as I did those in
the 24th Observation of the first Part of this Book. And when the Prism
was so placed as by refracting the Light of their mix'd Colours to
separate them, and distinguish the Rings from one another, as it did
those in that Observation, I could then see them distincter than before,
and easily number eight or nine of them, and sometimes twelve or
thirteen. And had not their Light been so very faint, I question not but
that I might have seen many more.

_Obs._ 5. Placing a Prism at the Window to refract the intromitted beam
of Light, and cast the oblong Spectrum of Colours on the Speculum: I
covered the Speculum with a black Paper which had in the middle of it a
hole to let any one of the Colours pass through to the Speculum, whilst
the rest were intercepted by the Paper. And now I found Rings of that
Colour only which fell upon the Speculum. If the Speculum was
illuminated with red, the Rings were totally red with dark Intervals, if
with blue they were totally blue, and so of the other Colours. And when
they were illuminated with any one Colour, the Squares of their
Diameters measured between their most luminous Parts, were in the
arithmetical Progression of the Numbers, 0, 1, 2, 3, 4 and the Squares
of the Diameters of their dark Intervals in the Progression of the
intermediate Numbers 1/2, 1-1/2, 2-1/2, 3-1/2. But if the Colour was
varied, they varied their Magnitude. In the red they were largest, in
the indigo and violet least, and in the intermediate Colours yellow,
green, and blue, they were of several