Francesco Maria Grimaldi
from The Supplement to the Penny Cyclopædia of the Society
for the Diffusion of Useful Knowledge (1851)
GRIMALDI, FRANCESCO MARIA, an Italian philosopher, and a member of the
order of Jesuits, was born at Bologna, in 1619. His education being
completed, he was, according to Montucla, employed during several years
in giving instruction in the belles-lettres; and during the latter part
of his life he applied himself to astronomy and optics. He died in
Bologna, in 1662, in the forty-fourth year of his age.
Grimaldi was associated with Riccoli in making astronomical
observations, and he gave particular descriptions of the spots on the
moon's disk. It was asserted by Montucla that Grimaldi gave to those
spots the designations by which they are now distinguished among
astronomers; thus superseding the names of the mountains and seas of
the earth which had been given to them by Hevelius: but this is
apparently a mistake.
That which has given celebrity to Grimaldi is his work entitled '
Physico-mathesis de Lumine, Coloribus et Iride aliisque annexis,' which
was published at Bologna in 4to., in 1665. The greater part of the work
consists of a tedious discussion concerning the nature of light, the
conclusion of which is that light is not a substantial but an
accidental quality; the rest, however, possesses the highest interest,
since it contains accounts of numerous experiments relating to the
interference of the rays of light. A description of the work in given
in the 'Philosophical transactions' for that year.
Grimaldi, having admitted the sun's light into a dark room, through a
small aperture, remarked that the breadths of the shadows of slender
objects, as needles and hairs, on a screem, were much greater than they
would have been if the rays of light had passed by them in straight
lines. He observed also that the circle of light formed on a screen by
the rays passing through a very small perforation in a plate of lead
was greater than it would be if its magnitude depended solely on the
divergency of the rays; and he arrived at the conclusion that the rays
of light suffer a change of direction in passing near the edges of
objects: this effect he designated 'diffraction.' By Newton it was
subsequently called 'inflexion.' He found that the shadow of a small
body was surrounded by three coloured streaks or bands which became
narrower as they receded from the centre of the shadow; and, where the
light was strong, he perceived similar coloured bands within the
shadow: there appeared to be two or more of these, the number
increasing in proportion as the shadow was farther from the body.
Having admitted the sun's rays into a room through two small circular
apertures, Grimaldi received the cones of light on a screen beyond the
place where they overlapped each other; and he observed, as might be
expected, that, within the space on which the rays from both apertures
fell, the screen was more strongly enlightened that it would have been
by one cone of light; but he was surprised to find that the boundaries
of the penumbral portions which overlaid one another were darker than
the corresponding portions in which there was no overlaying. This
phenomenon of interference was, at the time, enunciated as a
proposition:- 'That a body actually enlightened may become obscure by
adding new light to that which it has already received.'
Grimaldi also observed the elongation of the image, when a pencil of
light from the sun is made to pass through a glass prism: but he
ascribed the dispersion of light to irregularities in the material of
which the prism was formed; and he was far from suspecting the
different refrangibilities of the rays. The discovery of this fact,
which has led to so many important consequences in physical optics, was
reserved for Newton.
(Biographie Universelle; Montucla, Histoire des
Mathématiques.)
From The Supplement to the Penny Cyclopædia of the Society for the Diffusion of Useful Knowledge,
by George Long, publ. C. Knight (1851), pages 667-668.
Links
Today in Science History web page for Grimaldi day of birth, 2 April 1618.
