LXXII.] BLIND SPOT. 337 



Place in front of the eye of an albino rabbit a black screen with a hole in 

 it of exactly the same size as the pupil. Let the hole and pupil correspond 

 in position to each other. The pupil then appears black, as the card arrests 

 the lateral rays that fall upon the eyeball. 



16. The Pupil Appears Larger than it is in Reality. 



To see the pupil at its exact size, an excised eyeball must be observed in 

 water. If a glass model of a pupil be taken, and then be covered l.y an- 

 other thick concavo-convex glass in shape like the cornea, the pupil at once 

 appears larger. 



17. Lud wig's Apparatus for Vision of a Point. 



The black plate (fig. 253) is fixed in the slot so that either a slit or a hole 

 is just above the handle of the instrument. Remove from the instrument 

 the carrier with the steel point, and on tlie 

 bar of the instrument place the vertical 

 slit of the black plate (visual) near the 

 eye. There is a movable black j)late witli 

 a small hole in it. On looking at this 

 small hole through a vertical slit it appears 

 oval from above downwards, while with 

 a horizontal slit the round hole appears 

 drawn out laterally. If there be two small 

 holes near each other in the visual plate, 

 then at a certain distance two are seen in 

 the movable plate. If the m.ovable plate 

 be removed, and the steel point put in its 

 j.lace, on using the large hole in the visual ^^^- 253--;''.'dwig'8 Apparatus for 

 plate, and bringing the steel point towards ^'''*'" ""^ * ^'''"^• 



the eye, after a time one ceases to see it 



distinctly, or if seen it is blurred. On using the small hole in the visual 

 plate, the rod appears distinct (fig. 253). 



18. Listing's Reduced Eye. — The various dioptric media of the eye may be 

 considered as equal to a single substance with a refractive index of 1.35 and 

 a single spherical surface of radius 5.1248 mm. The position cf the nodal 

 point is 5 mm. behind the refit-active surface, and the i)rincipal focus 1 5 mm. 

 behind this. This latter value is of special importance in enabling one to 

 calculate the size of a retinal image — the size and distance of the object being 

 known. 



LESSON LXXII. 



BLIND SPOT — FOVEA CENTRALIS — DIRECT 

 VISION— CLERK-MAXWELL'S EXPERIMENT— 

 PHOSPHENES— RETINAL SHADOWS. 



1. The Blind Spot. 



(a.) Mamotte's Experiment. — As in fig. 254, on a white card, 

 make a cross and a large dot, either black or coloured. Hold the 

 card vertically about 10 inches from the right eye, the left bein^j 



