

o S 



DICTIONARY 



OF 



SCIENTIFIC TERMS. 



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A'BACUS (a/3af, a counter, a chess- 

 board). An ancient instrument for aid- 

 ing numerical calculations. The Greek 

 abacus consisted of an oblong frame, 

 having wires stretched across it, carrying 

 perforated ivory balls. In the Roman 

 abacus, the counters were slid along 

 grooves. 



ABBREVIATION {ahhrevio, to short- 

 en). An arithmetical term, denoting the 

 reduction of a fraction to lower terms, by 

 dividing the numerator and the denomi- 

 nator by any common factors which they 

 contain. Thus ^ divided by 5 gives 

 i||; and this, again divided by 3, gives 

 i|. The fraction is thus reduced or ab- 

 breviated, and, when it can no more be 

 reduced, it is said to be in its lowest 

 terms. 



ABDO'MEN {abdo, to conceal). The 

 posterior and principal cavity containing 

 the bowels and many other viscera of the 

 animal. The abdomen is distinct from 

 the thorax in the crustaceans, the spiders, 

 and the insects. 



ABDOMINA'LES {abdomen, the belly). 

 An order of malacopterygious or soft- 

 finned fishes, which have their ventral 

 fins placed on the abdomen, behind the 

 pectorals. The sub-families of the order 

 are termed by Cuvier, cyprinoides, silu- 

 roides, salmonoides, clupeoides, and lu- 

 cio'ides, from the respective typcal genera 

 of the carps, the silures, the salmons, the 

 herrings, andHhe pikes. 



ABDUCTOR MUvSCLE {abduco, to 

 draw from). A muscle whose office is to 

 draw one part of the body from another. 

 Thus the rectus externuvS is called ab 

 ductor oculi, fiom the action of this 

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muscle in drawing the eye away from the 

 nose. See Adductor. 



ABE'RRANT {aberro, to wander 

 from). A term applied in classification 

 to those species which deviate most from 

 the type of their natural group. 



ABERRATION Of LIGHT {aberro, 

 to wander from). An astronomical phe- 

 nomenon consisting in an alteration in 

 the apparent position of all the stars, 

 owing to the velocity with which light 

 moves, and to the motion of the earth in 

 her orbit, in a direction which forms a 

 tangent to the light from the heavenly 

 bodies. In consequence of the aberra- 

 tion resulting from the combined opera- 

 tion of these two velocities, all the fixed 

 stars appear to us to be rather more back- 

 ward than they really are in the direction 

 of the earth's annual motion. Aberra- 

 tion is never so much as 21"; in other 

 words, the apparent place of the star 

 differs from its real place less than the 

 ninetieth part of the apparent diameter 

 of the sun. 



1. Aberration, Spherical. A term em- 

 ployed in Optics to denote the deviation 

 of the rays of light from the true focus of a 

 curved lens or mirror ; in consequence of 

 which, instead of concentering in a single 

 point, they are spread over a certain sur- 

 face, forming a confused image of the 

 object. These aberrations proceed, l.from 

 the form of curvature of the lens or mir- 

 ror, and 2, from the different refrangibility 

 of the rays of light. See Refrangibility. 



2. Aberration, Chromatic. A term em- 

 ployed in Optics to denote the imperfec- 

 tion arising from the unequal refrangibi- 

 lity of the rays composing white light, 



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