

1814.] Refraction on Mechanical Principles. 21 



must be the same, and after being determined through the medium 

 must all tend to one and the same point. 



Let A B C D, rig. 4, represent a piano concave lens, whose 

 radius of curvature is E G ; E G Q will be the axis of the lens ; 

 any particle of light proceeding in this axis will, as before, continue 

 unchanged as to its direction ; but a particle of light, F L, will 

 touch the curve in the point v, and entering into a less resisting 

 medium will be deflected towards the perpendicular T E, and will 

 move in the direction z x, so at the side, C D, the particle enters 

 into a more resisting medium at the point S, the deflection from 

 the perpendicular h P will be increased, and it will move in the 

 direction x N, constituting a diverging ray. 



Newton has demonstrated that the velocity of light through diffe- 

 rent media is in the ratio of their refractive poweis ; consequently 

 the resistance must be in the same proportion diminished : all 

 transparent bodies with respect to light may be compared to con- 

 ducting bodies with respect to electricity ; they are conductors of 

 light ; and the first portion of light transmitted by any transparent 

 medium is that portion which was previously disseminated through 

 the substance, impelled by the light acting upon the opposite side : 

 so the spark we receive from the prime conductor of an electrical 

 machine is the electricity which previously existed in the conducting 

 body, impelled by the superinduced quantity, or more familiarly 

 may be compared to a tube of water, when more water is deter- 

 mined to one end of the tube, the first portion which passes through 

 the other orifice is that water which the tul>c previously contained. 

 Then the conducting powers of bodies for heat, for electricity, or 

 for light, are in nowise regulated by density : relative to light, we 

 know it is regulated by the refractive power, and this power is 

 greater in those substances which ate inflammable, and which are 

 hence supposed to contain the largest proportion of light ; and hence 

 probably some optical effects arise from that portion of light which 

 enters as a constituent part into the composition of combustible 

 bodies. 



\\ e hence may easily explain why scmipellucid bodies, such as 

 paper, are rendered transparent by being charged with substances, 

 such as oil. By the interposition of a transparent medium a con- 

 tinuum of light is produced. 



Dr. Priestley having observed that a particle of water floating over 

 a vegetable leaf, not only preserves a globular form, but also at its 

 interior portion exhibits a black spot, he hence deduced from the 

 17'h observation of the second book of Newton's Optics of '* Ob- 

 •ervationea circa Colorei Corporum tenuium pellucidorum," that 

 the panicle ol water is not in contact with the substance of the leaf, 

 but preserved hovering at a certain distance by a repulsive power. 

 \\ hen the leaf is examined by a lens of ± of an inch focus, the 

 explanation i» easy, the particle of water is suspended by the hairs 

 of the plant : if you break down the fibril las by your finger, no 

 such appearances of the water will take place. 



