LIGHT. 



fits of easy reflection ; and the returns in 

 the disposition to be transmitted, he calls 

 fits of easy transmission ; also the space 

 between the returns, the interval of the 

 fits. Hence then the reason why the sur- 

 faces of all thick transparent bodies re- 

 flect part of the light incident upon them, 

 and refract the rest, is, that some rays, at 

 their incidence, are in fits of easy reflec- 

 tion, and others of easy transmission. For 

 the properties of reflected light, see MIR- 

 ROR, OPTICS, &c. 



Again, a ray of light passing out of one 

 medium into another of different density, 

 and in its passage making an oblique 

 angle with the surface that separates the 

 mediums, will be refracted, or turned out 

 of its direction ; because the rays are 

 more strongly attracted by a denser, than 

 by a rarer medium- That these rays are 

 not refracted by striking on the solid parts 

 of bodies, but that this is effected without 

 a real contact, and by the same force by 

 which they are emitted and reflected, 

 only exerting itself differently in different 

 circumstances, is proved, in a great mea- 

 sure, by the same arguments by which it 

 is demonstrated that reflection is per- 

 formed without contact. 



When light is refracted by a prism, or 

 other transparent body, it is divided into 

 rays, exciting the sensation of different 

 colours ; namely, red, orange, yellow, 

 green, blue, indigo, and violet. This is 

 the enumeration followed by Newton and 

 others, which supposes seven rays re- 

 frangible in the above order, the red be- 

 ing least refrangible, and the violet most 

 so, and that the" other tints are produced 

 by mixture. The image formed by the 

 different rays, thus separated, forms the 

 solar spectrum. Dr. Wollaston has shown, 

 by looking through the prism at a narrow 

 line of light, that the primitive colours 

 are only red, green, blue, and violet. 



Heat and light are not present in cor- 

 responding degrees, in different parts of 

 the solar spectrum ; for, generally speak- 

 ing, those rays illuminate most that have 

 the least heating power. The rays in the 

 centre of the spectrum have the greatest 

 illuminating power, as may be ascertain- 

 ed by viewing, successively in each, a 

 small body, such as the head of a com- 

 mon nail. It will be seen most distinctly 

 in the light green, or deep yellow rays, 

 and less plainly towards either extremity 

 .)f the spectrum. 



The heating power of the rays follows 

 a different order. If the bulb of a sensi- 

 ble thermometer be moved, in succession, 

 through the differently coloured rays, it 



VOL. IV. 



will be found to indicate the greatest heat 

 in the red rays, next in the green, and so 

 on, in a diminishing progression, to the 

 violet. When the thermometer is re- 

 moved entirely out of the confines of the 

 red rays, but with its ball still in the line 

 of the spectrum, it rises even higher than 

 in the red rays ; and continues to rise, till 

 removed half an inch beyond the ex. 

 tremity of the red ray. The ball of the 

 thermometer employed for this purpose 

 should be extremely small, and should 

 be blackened with Indian ink An air 

 thermometer is better adapted than a 

 mercurial one, to exhibit the minute 

 change of temperature that ensues. These 

 invisible heat-making rays may be reflect- 

 ed by the mirror, and refracted by the 

 lens, exactly in the same manner as the 

 rays of light. 



Beyond the confines of the spectrum on 

 the other side, viz. a little beyond the vio- 

 let ray, the thermometer is not affected ; 

 but in this place it is remarkable, that 

 there are also invisible rays of a different 

 kind, which exert all the chemical effects 

 of the rays of light, and even with greater 

 energy. One of the chemical properties 

 of light is, that it speedily changes from 

 white to black the fresh precipitated 

 muriate of silver. This effect is pro- 

 duced most rapidly by the direct light ot 

 the sun ; and the rays, as separated by 

 the prism, have this property in various 

 degrees. The blue rays, for example, 

 effect a change of the muriate of silver in 

 fifteen seconds, which the red require 

 twenty minutes to accomplish ; and, ge- 

 nerally speaking, the power diminishes as 

 we recede from the violet extremity. But 

 entirely out of the spectrum, and beyond 

 the violet rays, the effect is still pro- 

 duced. Hence it appears that the solar 

 beams consist of three distinct kinds of 

 rays; of those that excite heat, and pro- 

 mote oxydation ; of illuminating rays ; 

 and of de-oxydizing rays. A striking 

 illustration of the different power of these 

 various rays, is furnished by their effect 

 on phosphorus. In the rays beyond the 

 red extremity, phosphorus is heated, 

 smokes, and emits white fumes ; but 

 these are presently suppressed on expos- 

 ing it to the de-oxydizing rays which lie 

 beyond the violet extremity. 



There i.s an exception, however, as 

 stated by Dr. Wollaston, to the c!e-oxydiz- 

 ing power of the rays above-mentioned. 

 The substance, termed gum-guiacum, has 

 the property, when exposed to the light, 

 of changing from a yellowish colour to 

 green; and this cfTeot. he has ascertained 



Q 



