. The microscope and its revelations. 412 MANIPULATION AND PRESERVATION OF THE MICROSCOPE which constitiite our daylight fall from every point of the visibleheavens (though with greatly diminished intensity). That is to say,we have at disposal a light source extending over 180°, while the sunitself extends over a visual angle of bat half a degree. Being thussurrounded by an illimitable and self-luminous expanse of ether un-dulations, the question is no longer of parallel rays only, but of lightemanating from an outer circle above the earth upon every point ofthe earths surface ; and a mirror e
. The microscope and its revelations. 412 MANIPULATION AND PRESERVATION OF THE MICROSCOPE which constitiite our daylight fall from every point of the visibleheavens (though with greatly diminished intensity). That is to say,we have at disposal a light source extending over 180°, while the sunitself extends over a visual angle of bat half a degree. Being thussurrounded by an illimitable and self-luminous expanse of ether un-dulations, the question is no longer of parallel rays only, but of lightemanating from an outer circle above the earth upon every point ofthe earths surface ; and a mirror exposed to such a luminous atmo-sphere must both receive and reflect from all sides and upon allsides. If, however, it be placed under the stage of a microscope,all vertical light is intercepted, and there remains nothing but theoblique incidence as the starting-point of the theory of illuminationby converging light ; for it scarcely needs repetition that obliquityof incidence gives inevitable rise to obliquity of reflexion ; and it. S FIG. 353.—Light from the open sky falls upon the mirror in all directions. becomes equally clear that in order to strike the object themust al/waysfall obliquely on the mirror. Then it follows from what has been said that the light fallingfrom the open sky upon a mirror falls in all conceivable fig. 353 shows the lines 1 to 7, including an angle of 30°. Ifnothing intervene, the light of that sky surface must fall upon themirror, a b, and be reflected on O. The intermediate rays, 2, 3, 4,5, 6, form the converging Ultnnnndnnj /><//cil, with of course an in-finity of others filling up the spares between. In other words, every point of a mirror is a radiant of a wholehemisphere, and this is equollt/ l//f K hctlicr the. mirror be. plmn,concave, or convex, so long as it is exposed to a boundless a plane, concave, or convex mirror will give a cone of LIGHT REFLECTED TO A FOCUS FROM THE OPEN SKY 413
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
