The microscope and its revelations . iU then be visible through it, ifthe eye be placed beyond the limiting angle; whilst the surface itself wiUappear, as if silvered, through its reflecting back to the eye the light whichfalls upon it from beneath. 40 OPTICAL PRINCIPLES OF THE MICROSCOPE. is rendered more exact (§§10,12).—It is easily shown to be inaccordance with, the laws of refraction already cited, that whena pencil of parallel rays, passing through air, impinges upona convex surface of glass, the rays will be made to converge;for they will be bent towards the centre of the circle, therad
The microscope and its revelations . iU then be visible through it, ifthe eye be placed beyond the limiting angle; whilst the surface itself wiUappear, as if silvered, through its reflecting back to the eye the light whichfalls upon it from beneath. 40 OPTICAL PRINCIPLES OF THE MICROSCOPE. is rendered more exact (§§10,12).—It is easily shown to be inaccordance with, the laws of refraction already cited, that whena pencil of parallel rays, passing through air, impinges upona convex surface of glass, the rays will be made to converge;for they will be bent towards the centre of the circle, theradius being the perpendicular to each point of central or axial ray, as it coincides with the perpendicular,will undergo no refraction; the others will be bent from theiroriginal course ui an increasing degree, in proportion as theyfall at a distance from the centre of the lens ; and the effectupon the whole will be such, that they ?^dU be caused to meetat a point, called i^iQ focus, some distance beyond the centre. Parallel rays, falling on a plano-convex lens, brought to a focus atthe distance of its diameter; and conversely, rays diverging fromthat point, rendered parallel. of curvature.—This effect wiU not be materially changed, byallowing the rays to pass into air again through a plane surfaceof glass, perpendicular to the axial ray (Fig. 1); a lens of thisdescription is called a plano-convex lens; and it wiH hereafterbe shown to possess properties, which render it very useful inthe construction of microscopes. But if, instead of passingthrough a plane surface, the rays re-enter the air through asecond convex surface, turned in the opposite direction, as ina double-convex lens, they wiU be made to converge still will be readily comprehended, when it is borne in mindthat the contrary direction of the second surface, and the con-trary direction of its refraction (this being/rom the densermedium, instead of i7ito it), antagonize each other; so that
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