The London dispensatory : containing I. The elements of pharmacy, II. The botanical description, natural history ... of the materia medica, III. The pharmaceutical preparations and compositions of the Pharmacopoeias of London, Edinburgh and Dublin the whole forming a practical synopsis of materia medica, pharmacy and therapeutics / by Anthony Todd Thomson.

  • Anthony Todd Thomson
Date:
1833
Catalogue details

Licence: Public Domain Mark

Credit: The London dispensatory : containing I. The elements of pharmacy, II. The botanical description, natural history ... of the materia medica, III. The pharmaceutical preparations and compositions of the Pharmacopoeias of London, Edinburgh and Dublin the whole forming a practical synopsis of materia medica, pharmacy and therapeutics / by Anthony Todd Thomson. Source: Wellcome Collection.

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    with the fact, that a ray of light, which has suffered ordinary refraction in passing through a crystal in one direction, should suffer ordinary refraction in passing through it in another direction; whilst this fact perfectly accords with the idea that light consists of particles endowed with rectilineal motion. The size of the particles of light is too minute to be appre¬ ciated ; but their velocity is estimated to be at the rate of 200,000 miles in a second. They appear to repel each other like the particles of caloric, A ray of light falling obliquely upon a polished surface is reflected from it at an angle equal to the angle of its incidence. But when a ray of light falls at an angle of 35° 25' on a polished plate of glass; and, in being reflected from it, falls upon another plate of glass, so placed that its angle of in¬ cidence is also 35° 25', the second plate may be turned round its axis without varying the angle which it makes with the ray that falls upon it. If the two planes of reflection be parallel to each other, the ray of light is reflected in the same manner from both plates of glass; but if the second plate be turned round a quadrant of a circle, so as to make the plane of re¬ flection perpendicular, the whole ray will pass through it, and none of it be reflected; yet, if this plate be turned round another quadrant of a circle, so as to make the reflecting planes again parallel, the ray will be now reflected by it as at first. The light can penetrate through the glass only when the reflecting planes are perpendicular, but is reflected when they are parallel. This property of light has been termed by Malus, by whom it was first discovered, its polarization. When the ray of light, moving in a straight line, passes within a certain distance of a body parallel to its direction, it bends towards the body, or is inflected; but when the body parallel to its course is at a greater distance, the ray is bent from it, or deflected. When it passes obliquely from one me¬ dium to another of a different density, it is bent a little from the line of its former direction, and assumes a new one, or is refracted. In passing into a denser medium it is refracted towards the perpendicular ; but is refracted from the per¬ pendicular when passing into a rarer medium. The refraction is proportional to the density of the medium ; but, in that of a combustible, the refraction is greater than the ratio of its density; and, when a liquid is converted into vapour, its refractive power diminishes at a greater ratio than its density diminishes.1 Every ray of light is resolvable into seven other distinct rays, each possessing a different degree of refrangibility; and 1 See the experiments of Arago and Petit, in the Ann. de Chim, ct JPhys. tom. i. ].