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	|  | Figure 20-1: Refraction of light as it enters a transparent medium of higher refractive index n′ | 
	|  | Figure 20-2: Example of the laws of reflection and refraction. | 
	|  | Figure 20-3: Example of the critical angle. | 
	|  | Figure 20-4: Illustration of three rays traced in trigonometric ray tracing. | 
	|  | Figure 20-5: Algebraic thin lens approximations. All lengths in meters. | 
	|  | Figure 20-6: Description of a thick lens in Gaussian optics. | 
	|  | Figure 20-7: Ray tracing through plus and minus lenses. | 
	|  | Figure 20-8: Chromatic aberration of lenses. | 
	|  | Figure 20-9: Spherical aberration of a biconvex lens. | 
	|  | Figure 20-10: A planocylindrical lens with axis in the horizontal meridian. | 
	|  | Figure 20-11: Top: Illustration of prism base notation. Bottom: Illustration of cylinder axis notation. | 
	|  | Figure 20-12: The conoid of Sturm, formed by light refracted by an astigmatic lens. | 
	|  | Figure 20-13: Cross diagram and equivalent combinations, including longhand notations, for a spherocylindrical lens. | 
	|  | Figure 20-14: Calibration of prisms. Glass prisms and spectacle prisms are calibrated according to the Prentice position, whereas plastic prisms are calibrated according to the position of minimum deviation. | 
	|  | Figure 20-15: Example of the prism as used in ophthalmology. | 
	|  | Figure 20-16: Power of a prism in prism diopters. | 
	|  | Figure 20-17: The Fresnel prism. | 
	|  | Figure 20-18: The reduced schematic eye. | 
	|  | Figure 20-19: The optical system of the eye, illustrating the three-lens concept. | 
	|  | Figure 20-20: Snellen block E. | 
	|  | Figure 20-21: Spherical refractive errors as determined by the position of the secondary focal point with respect to the retina. | 
	|  | Figure 20-22: Types of regular astigmatism as determined by the positions of the two focal lines with respect to the retina. | 
	|  | Figure 20-23: Types of astigmatism as determined by the orientation of the principal meridians and the orientation of the correcting cylinder axis. | 
	|  | Figure 20-24: The retinoscopic reflex. | 
	|  | Figure 20-25: Movement of the retinoscopic reflex. |