Bryn Mawr College monographs . be taken not to place the fixation-point so far from the stimulus-opening that the color used is seen of a different quality at this point than at thestimulus-opening. If this should occur for any color when the fixation is at the 25point, a lesser excentricity should be chosen. The sensitivity of the contrast-effectto change of illumination is not lessened to any considerable extent, for example, bytaking a 20° instead of a 25° fixation-point. We have found the 25° point to be very METHOD OF PHOTOMETRY plane normal to the screen midway between the stimulus-openi
Bryn Mawr College monographs . be taken not to place the fixation-point so far from the stimulus-opening that the color used is seen of a different quality at this point than at thestimulus-opening. If this should occur for any color when the fixation is at the 25point, a lesser excentricity should be chosen. The sensitivity of the contrast-effectto change of illumination is not lessened to any considerable extent, for example, bytaking a 20° instead of a 25° fixation-point. We have found the 25° point to be very METHOD OF PHOTOMETRY plane normal to the screen midway between the stimulus-opening and the fixation-point. The contrast-stimulus andthe measuring disc thus receive equal amounts of light fromthe source to be measured. In Fig. I is given a picture of the apparatus in the roughform in which it is at present used. At 5 is shown the screenwhich is illuminated in turn by the standard and comparisonlights; at B the photometer bar; at D the measuring disc;at L the lamp to be photometered; at M the mouthboard;. Fig. I and at 0 the opening filled with gray which is darkened ingraded amounts by induction from the screen S as theamount of light received by it from the lamp L is screen 5 is supported by short rods screwed into heavytripod bases. In order to provide for adjusting the heightof the screen above the table the frame of the screen isfastened to the supporting rods by means of a collar and setscrew. On the upper edge of this frame is attached a devicefor lining up the eye with the stimulus-opening 0. Thisdevice consists of the following parts: A vertical arm carryingat Its lower end a small circle 15 mm. in diameter extends satisfactory, however. For a fixation-point we used a black knot on a gray thread,stretched from a point on the screen S to a rod in front and on the opposite side ofthe measuring disc. o FERREE AND RAND down to the level of the stimulus-opening 0. Attached tothe vertical arm just above this circle is a horizontal arm15 cm.
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Keywords: ., bookauthorbrynmawr, bookcentury1900, bookdecade1900, bookyear1901
