. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. 5o LIGHTING AND FOCUSING. [CH. II. great advantage. This combination is generally taken advantage of in histology. Fig. 89 is an example of a purely refractive 53. N>. 54- N\ 55- N>. Figs. 53-55.—Diagrams illustrating refraction in different media and at plane and curved surfaces. In each case the denser medium is represented by line shad- ing and the perpendicular or normal to the refracting surface is represented by the dotted line N-N', the refracted ray by the bent line A C. \ 93. Re
. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. 5o LIGHTING AND FOCUSING. [CH. II. great advantage. This combination is generally taken advantage of in histology. Fig. 89 is an example of a purely refractive 53. N>. 54- N\ 55- N>. Figs. 53-55.—Diagrams illustrating refraction in different media and at plane and curved surfaces. In each case the denser medium is represented by line shad- ing and the perpendicular or normal to the refracting surface is represented by the dotted line N-N', the refracted ray by the bent line A C. \ 93. Refraction.—Lying at the basis of microscopical optics is refraction, which is illustrated by the above figures. It means that light passing from one medium to another is bent in its course. Thus in Fig. 53, light passing from air into water does not continue in a straight line but is bent toward the normal N-N', the bending taking place at the point of contact of the air and water ; that is, the ray of light A B entering the water at B is bent out of its course, extending to C in- stead of to C. Conversely, if the ray of light is passing from water into air, on reaching the air it is bent from the normal, the ray C B passing to A and not in a straight line to C". By comparing Figs. 54, 55, in which the denser medium is crown glass in- stead of water, the bending of the rays is seen to be greater as crown glass is denser than water. It has been found by physicists that there is a constant relation between the angle taken by the ray in the rarer medium, and that taken by the ray in the denser medium. The relationship is expressed thus : Sine of the angle of incidence di- vided by the sine of the angle of refraction equals the index of refraction. In the figures, _—'^ o Af/ = index of refraction. Worked out completely in Fig. 53, Sin CBN' A B N = 4o°, CB N'=28° 54'and- Sin 40° 0,64279 = i-33. ,, 1. <?., the index of In Figs. 54-55, illustrating refrac- Sin
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