COL 



COL 



glass, is called in Astronomy its line of 

 collimation. The difference between the 

 existing and the required positions of an 

 instrument is called the error of collima- 

 tion. This error is determined, without 

 reversal of the instrument, by means of 

 a contrivance called a collimator. 



CO'LLUM. The neck. This term de- 

 notes, in Botany, that portion of the axis 

 of growth where the stem and the root 

 diverge : by Grew it was termed coarc- 

 ture ; by Lamarck, vital knot. 



CO'LOPHONITE. A mineral with a 

 resinous fracture, found in magnetic 

 iron-stone in Norway, identical with 

 the resinous garnet of Haiiy and Jame- 

 son. 



CO'LOPHONY. Resin of turpentine ; 

 the residue of common turpentine, after 

 its essence has been removed by distilla- 

 tion. It is not a homogeneous product, 

 but has been divided into two different 

 resins, named by Unverdorben the sylvic 

 and pinic acids. 



COLOURS. A general term for those 

 modifications of light, whether direct or 

 reflected from other bodies, by which the 

 sight is affected with distinct sensations. 

 The colour of a body is designated by the 

 particular species of light reflected from 

 its surface, which may therefore be called 

 the objective colour, or that which is per- 

 manent to the body. 



1. Prismatic Colours. Colours pro- 

 duced by transmitting white light through 

 colourless prismatic bodies. The light 

 undergoes refraction, and is dispersed, 

 presenting the phenomenon of the solar 

 or prismatic spectrum. The colours ex- 

 hibited by opaque or non-luminous sub- 

 stances, when they either reflect or trans- 

 mit white light, are called natural 

 colours. 



2. Primary Colours. These are red, 

 orange, yellow, green, blue, and violet, 

 being the different colours into which a 

 solar ray of light may be decomposed. 

 White is, therefore, a compound of these 

 colours, and black is the absence of all 

 colours. The primary colours are not 

 decomposable, but have uniform refrac- 

 tion ; hence they were termed by Newton, 

 homogeneous colours, to distinguish them 

 from common or white light, which he 

 called compound or heterogeneous. 



3. Secondary Colours. According to 

 Brewster, there are only three primary 

 ot fundamental colours; viz. red, yellow, 

 and blue; the other three, viz. orange, 

 green, and violet, being produced by 

 combination of the former, and therefore 



84 



properly termed secondary or compound 

 colours. 



4. Complementary Colour. That colour 

 with which each of the prismatic colours 

 will combine, and form white light. The 

 complementary colour to each of the 

 three primaries, is the compound colour 

 obtained by combining the other two pri- 

 maries ; and, for each of the secondaries, 

 that primary one which does not enter 

 into its composition. 



5. Accidental Colours. A series of 

 optical phenomena, commonly called 

 ocular spectra. If the eye be steadily 

 directed, for some time, to a white wafer 

 upon a dark ground, and be then turned 

 aside, a spectrum of the wafer will be 

 perceived, with the colours reversed : the 

 wafer will appear dark, the ground white. 

 If the wafer be red, its spectrum will be 

 bluish-green ; if orange, blue ; if blue, 

 orange-red, &c. Darwin classes the 

 spectra under the heads of direct and re- 

 verse, the former depending on the per- 

 manence of the impression, the latter 

 upon exhaustion. Accidental colours 

 are sometimes called subjective or phy- 

 siological, to distinguish them from the 

 permanent or objective colours. 



6. Entoptic Colours. A term applied 

 by Seebeck to the spectra obtained by 

 transmitting polarized light through a 

 crystal cut at right angles to its axis, the 

 light being made to pass in the direction 

 of the axis. The phenomena consist of 

 concentric rings of various colours, re- 

 sembling Newton's prismatic rings, and 

 intercepted in the middle by a rect- 

 angular cross. 



COLOURED RINGS. Luminous 

 rings produced by interference of the 

 rays of light, or by electro-chemical de- 

 composition. Of the former kind are 

 Newton's coloured rings, which may be 

 produced by pressing a watch-glass upon 

 a piece of plate-glass ; the rings will ap- 

 pear about their point of contact, of the 

 same colour as the incident rays, if the 

 light be homogeneous. Of the second 

 kind are Nobili's rings, which are pro- 

 duced when a feeble current of electricity 

 is directed upon a polished metallic plate, 

 this plate being used as an electrode, 

 and immersed in an electrolytic fluid; 

 the rings will then appear upon the 

 plate. 



COLOURING MATTER. Colouring 

 principles found in vegetable substances, 

 and employed in dyeing. Colours are 

 termed substantive, when they adhere to 

 the cloth without the intervention of a 



