Ocular refraction and the shadow test . Figure 84. lliibroken appearance of a vertical straightpositions. The cylinder r Mark with India ink at the edge of the lens the four pointsthrough which the line appears to pass, unbroken, this procedurebeing similar to that in experiment illustrated by figure 47. It willbe found that these points are 90° apart, that if opposite points bejoined by straight lines, these lines will cross each other at rightangles. If they do not, erase them and try again for this is the proofthat the points are correctly located. These two lines •ill iiidieate the two
Ocular refraction and the shadow test . Figure 84. lliibroken appearance of a vertical straightpositions. The cylinder r Mark with India ink at the edge of the lens the four pointsthrough which the line appears to pass, unbroken, this procedurebeing similar to that in experiment illustrated by figure 47. It willbe found that these points are 90° apart, that if opposite points bejoined by straight lines, these lines will cross each other at rightangles. If they do not, erase them and try again for this is the proofthat the points are correctly located. These two lines •ill iiidieate the two piiiieil^a! iiieriiUaiis of tliecylinder, an,I their point of interseetion ivill be its opiieal centre. This experiment serves a double purpose, by means of it we canidentify a cylinder and locate its principal meridians. The next pointis to determine which of these is the principal meridian and which the 82 OCULAR REFRACTION. axis. Rotate the plane cylinder to a position in which the verticalstraight line appears through it unbroke
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