Ocular refraction and the shadow test . ay, tin, having a very small aperture at A A; the screen ofwhite cardboard I is placed back of the plate to receive the R from all points of the candle meet the plate P and are inter-cepted by it, only those reaching A A pass through and are received bythe screen I; if A A were small enough only one ray from each pointwould reach I. As the paths of the rays are straight, a ray from eachof the points A, B and C reach the screen at the points A, B and C;the same is true of all rays forming the image and this shows why theimages formed by apertur
Ocular refraction and the shadow test . ay, tin, having a very small aperture at A A; the screen ofwhite cardboard I is placed back of the plate to receive the R from all points of the candle meet the plate P and are inter-cepted by it, only those reaching A A pass through and are received bythe screen I; if A A were small enough only one ray from each pointwould reach I. As the paths of the rays are straight, a ray from eachof the points A, B and C reach the screen at the points A, B and C;the same is true of all rays forming the image and this shows why theimages formed by apertures are inverted. If the distance of thescreen from the aperture is equal to the distance of the candle fromthe aperture, the image will be the same size as the candle; if furtheraway, say at I, the image A B C will be larger than the object;the nearer the screen is brought to the aperture the smaller andbrighter will be the image. If the aperture is made through a thickplate the image formed will be poor. To obtain a clear cut image it. Figure 17. Formation of image by an aperl is necessary to have the smallest possible aperture in the thinnestpossible plate. A camera can be constructed upon this principle andphotographs made without the use of a lens, which in point of detailare superior to those made with a lens. The greatest drawback tosuch pictures is the time required for the exposure (.ue to the smallamount of the light projected upon the plate. Any number of aper- O C U L A K E 1- R A C T I tures can be used in the expsriment illustrated by figure 17, and eachwill create an imag-e which will be distinct so long as it is not over-lapped by the image from another aperture; when the apertures areso close together that the images they create overlap, the result is illu-mination and no formation of distinct images. Figure 18 represents a plane mirror A B, to the eye placed at Ethe image of the candle C appears by reflection at C, which is thesame size and shape, in other word
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