Ch. V] 



THE MICROSCOPE AND VISUAL ANGLE 



129 



Or to put it in another way, the microscope helps the eye to produce 

 a larger retinal image, and makes the details large enough to fall on 

 more than one of the retinal elements, thus making resolution possible. 



The sensory receptors of the retina — the rods and cones — are quite 

 close together and over the greater part of the retina are commingled, 

 there being more rods 

 than cones. In the re- 

 gion of greatest visual 

 acuity (fovea centralis 

 of macula lutea), only 

 cones are present. In 

 general the rods are 2 //. 

 and the cones 6 /x in di- 

 ameter. In the fovea, 

 however, the cones are 

 slender, being only 

 about 2 //, or 3 ju, in di- 

 ameter. These sizes 

 give a clue to the size 

 the retinal image must 

 have in order that there 

 be resolution, that is, 

 that two points appear 

 as two or two hnes ap- 

 pear as two. 



If we assume that 

 Hooke was correct in 

 the assumption that for 

 two points to appear as two a visual angle of i minute is necessary, 

 the diameter in millimeters or inches of the object, or the separation 

 of the two points to render them visible as two, is easily determined 

 as foUows: 



The nodal point or optic center of the eye is considered to be at 

 the center of a circle (fig. 75), and the object at the circumference. 

 No matter how great or how small the visual distance, the object must 

 subtend one minute of the arc of the circle at whose circumference it 



Fig. 76. Constant Size of Object, the Vis- 

 ual Angle and the Retinal Image Varying 

 with the Distance. 



R I The retinal image varying inversely as 

 the distance of the object. 



V A The visual angle varying with the dis- 

 tance of the object from the eye. 



Object The object of constant size but varying 

 distance from the eye. The distance of the object 

 is in the ratio of i, ;:, 4. The entire circle is 

 shown at the right, but only a small arc in the 

 other figures. 



