FOL 



FOR 



the hyperbola, where the radii from all 

 parts of these curves meet. 



1. Foci of an Ellipse. Two points in 

 the transverse diameter of an ellipse, 

 equally distant from its ends, and so 

 situate that, if any point be taken in the 

 circumference of the ellipse, and lines 

 drawn from that point to the two foci, 

 the length of these lines, when joined 

 together, will always be the same, at 

 ■whatever part of the circumference the 

 point may be taken. 



2. Foci of Mirrors. 1. The focus of a 

 concave mirror is that point of the axis 

 through which the reflected rays of light 

 pass ; the principal focus is the focus of 

 parallel rays. 2. In the convex mirror, 

 the focus lies as far behind the reflecting 

 surface as in the concave it lies before 

 it; it is generally callt d the virtual focus, 

 because it is only an imaginary point, 

 and is not formed by the actual union of 

 rays in a focus. Similar terms are em- 

 plo3'ed in speaking of the foci of lenses. 



3. Focus, Geometrical and Refracted. 

 The point in which the rays of light, ac- 

 cording to their known laws, ought to be 

 concentrated, when reflected from a con- 

 cave mirror, or refracted through a lens, 

 is termed the geometrical focus ; that in 

 which they are actually found, is the 

 refracted focus. These foci are separated 

 from each other in proportion to the 

 degree of spherical aberration. 



4. Focal Distance. In the concave 

 mirror, this is the distance between the 

 focus and the vertex of the mirror, and 

 it is always equal to half the radius of 

 that sphere of which the mirror forms a 

 segment. In the convex lens, it is the 

 distance of the focus from the surface of 

 the glass, or from the optical centre. 



FO'LIATE CURVE {folium, a leaf). 

 A curve line of the third order, defined 

 by the equation x^+y^=axy. It is one 

 of the species of defective hyperbolas, 

 having one asymptote and two infinite 

 branches. Its name is derived from the 

 resemblance of its figure to that of a leaf. 



FO'LIATED (folium, a leaf). Resem- 

 bling a leaf. The external surface of a 

 shell is said to be foliated, when its va- 

 rices or spines are divided at their mar- 

 gins into leaf-like segments, as in the 

 rose-bush murex. The substance of a bi- 

 valve shell is also said to be foliated, 

 when it consists of thin flat plates 

 overlapping each other. 



FO'LIATED COAL [foliatus, having 

 the texture of leaves). A sub-species of 

 black coal, occurring in the coal forma- 

 140 



tions, and distinguished by its lamellar 

 concretions, splendent lustre, and easy 

 frangibility. 



FOLIATION {folium, a leaf). A terra 

 synonymous with vernation, denoting the 

 manner in which the young leaves of 

 plants are arranged in the leaf-bud. The 

 terms expressive of the various modes of 

 arrangement of leaves are the same as 

 those employed for the floral envelopes, 

 and will be found under the word Esti- 

 vation. 



FO'LIOLE (foliolum, a little leaO- A 

 leaflet; a term applied, in Botany, to the 

 small leaves which are borne on the 

 several divisions of what is commonly 

 called a compound leaf. 



FO'LLICLE (dim. of follis, a pair of 

 bellows). Literally, a little bag, or scrip 

 of leather. The term denotes, in Botany, 

 a one-celled, one-valved, superior fruit, 

 dehiscent along its face, as in paeonia. 



Double Follicle. This term is applied 

 by Mirbel to the conceptaculum of other 

 writers, and consists of a two-celled, su- 

 perior fruit, separating into two portions, 

 the seeds of which do not adhere to mar- 

 ginal placentae, as in the follicle, but 

 separate from their placentae, and lie 

 loose in each cell, as in asclepias. 



FO'MALHAUT. A star of the first 

 magnitude in the southern constellation 

 Fiscis Australis. 



FORA'MEN (foro, to pierce). An 

 opening. A passage observed at the 

 apex of the ovule in plants, and com- 

 prising both the exostome and the en- 

 dostorae. 



FORAMINI'FERA {foramen, an open- 

 ing, fero, to bear). The name of a family 

 of microscopic shells, the chambers of 

 which communicate with each other by 

 a small opening or foramen. They are 

 not all cephalopods, as D'Orbigny sup- 

 posed, who gave them the name. 



FORCE. The general designation of 

 any exertion which has a tendency to 

 move a body at rest, or to affect or stop 

 the progress of a body already in motion. 

 This is sometimes termed active force, in 

 contradistinction to that which merely 

 resists or retards the motion of a body, 

 but is itself, apparently, inactive. The 

 degree of resistance to any motion may 

 be measured by the active force required 

 to overcome that resistance, and hence 

 writers on Mechanics make use of the 

 terms resisting forces and retarding forces. 

 The straight line in which any force 

 tends to make a body move, is termed 

 the line of direction of the force. 



