James Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections the spherical aberration is corrected.
In grating spectroscopes both plane and concave gratings are employed in connexion with a collimator and observing telescope.
The umbrella-like body bears a circle of tentacles at the edge, whereby the body can be divided into a convex exumbrella or exumbral surface and a concave subumbrella or subumbral surface.
The swiftest current te,-ids, by reason of centrifugal force, to follow the outer side of every significant curve in the channel; hence the concave bank, against which the rapid current sweeps, is worn away; thus any chance irregularity is exaggerated, and in time a series of large serpentines or meanders is developed,, the most-symmetrical examples at present being those near Greenville, Miss.
This was done in 1612 by Christoph Scheiner, who fully described his method of solar observation in the Rosa Ursina (1630), demonstrating very clearly and practically the advantages and disadvantages of using the camera, without a lens, with a single convex lens, and with a telescopic combination of convex object-glass and concave enlarging lens, the last arrangement being mounted with an adjustable screen or tablet on an equatorial stand.
One was a wooden box with a projecting tube in which a combination of a concave with a convex lens was fitted, for throwing an enlarged image upon the focusing screen, which in its proportions and application is very similar to our modern telephotographic objectives.
Rayleigh points out that this clinging of the sound to the surface of a concave wall does not depend on the exactness of the spherical form.
So closely allied are these two fishes that their distinctness can be proved only by an examination of the gill-apparatus, the allis shad having from sixty to eighty very fine and long gill-rakers along the concave edge of the first branchial arch, whilst the twaite shad possesses from twenty-one to twenty-seven stout and stiff gill-rakers only.
Kepler, who examined Porta's account of his concave and convex lenses by desire of his patron the emperor Rudolph, declared that it was perfectly unintelligible.
Nearly related is the extinct family Lophiodontidae (inclusive of the American Helaletidae), in which both the upper and lower first premolar may be absent, while the upper molars present a more rhinoceros-like form, owing to the lateral compression and consequent lengthening of the outer columns, of which the hinder is bent somewhat inwards and is more or less concave externally, thus forming a more complete outer wall.