. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . the lensL2 and are brought to a focus on the screen DD. In the screen thereis a second slit S2 through which rays of only one refrangibility rays fall on a third lens L3 arranged so as to produce on awhite screen at FE an image of the nearer face of the prism. Bymoving the slit S2 a patch of light of any required color can be thrownon the screen at FE. Slight tiltings of the lenses L2 and L3 must begiven in order that a sharp image of the whole of the prism face maybe formed on FE. To
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . the lensL2 and are brought to a focus on the screen DD. In the screen thereis a second slit S2 through which rays of only one refrangibility rays fall on a third lens L3 arranged so as to produce on awhite screen at FE an image of the nearer face of the prism. Bymoving the slit S2 a patch of light of any required color can be thrownon the screen at FE. Slight tiltings of the lenses L2 and L3 must begiven in order that a sharp image of the whole of the prism face maybe formed on FE. To apply this device and method to color photometry it is necessarythat a vertical stick be placed in the path of this colored beam castinga shadow on the screen, while a second standard light T2 mounted on 243 PHYSIOLOGICAL OPTICS a scale, casts a shadow close by. This second shadow is colored, beingilluminated by the colored beam from S2, while the first shadowreceives the light from the standard; still, by moving the comparisonlamp along the scale a point can be found at which the luminosities. Fig. 124.—Abneys Apparatus for Obtaining Luminosity Curves of the Eye. over the two appear equal. The determination of this point is, how-ever, attended with difficulty much of which is overcome by theoscillation method described in 1886 by Abney and Festing. Abneyfound That the best way of determining the intermediate point / s- v; ? / A°- < 53 / to \r / \m \ \ v \ s., sa 56 54- 5Z SO 4-8 46 4* too90 80 70SO5040302o10 o o ?z. z ?=3 V/OLET BLUE GREEN ORANGE RED Fig. 125.—Luminosity Curves of a Normal and a Bed Color Blind Eye. (After Abney.) where the shadows balance is by oscillating the slide gently betweentwo points when first one shadow and then the other is palpably toodark; the oscillations become shorter and shorter until the point ofbalance is determined. By pursuing this method throughout thewhole spectrum Abney obtained the curves shown in Fig. 125. Thefull line curve
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