THE EYELIDS. 793 



the glass plates. In order to see b 1} the eyes (?i and q) must diverge more, so that b appears 

 more distant and larger. 



3. In looking through Wheatstone's reflecting stereoscope (fig. 571), it is obvious that the more 

 the two images approach the observer, the more must the observer converge his visual axes, 

 because the angles of incidence and reflexion are greater. Hence the compound picture now 

 appears to him to be smaller. If the centre of the image, R, recedes to R x , then of course the 

 angle, S n , rp, is equal to S lt rR lf and the same on the left side. 



4. In using the telestereoscope, the two eyes are, as it were, separated from each other, then 

 of course in looking at objects at a certain distance the convergence of the visual axes must be 

 greater than in normal vision. Hence, objects iii a landscape appear as in a small model. But 

 as we are accustomed to infer that such small objects are at a great distance, hence the objects 

 themselves appear to recede in the distance. 



Estimation of Distance. When the retinal images are of the same size, we 

 estimate the distance to be greater the less the effort of accommodation, and con- 

 versely. In binocular vision, when the retinal images are of the same size, we 

 infer that that object is most distant for which the optic axes are least converged, 

 and conversely. Thus, the estimation of size and distance go hand in hand, in 

 great part at least, and the correct estimation of the distance also gives us a correct 

 estimate of the size of objects (Desca?tes). A farther aid to the estimation of 

 distance is the observation of the apparent displacement of objects, on moving our 

 head or body. In the latter, especially, lateral objects appear to change their 

 position toward the background, the nearer they are to us. Hence, when travel- 

 ling in a train, in which case the change of position of the objects occurs very 

 rapidly, the objects themselves are regarded as nearer, and also smaller (Dove). 

 Lastly, those objects appear to us to be nearest which are most distinct in the field 

 of vision. 



Example. A light in a dark landscape, and a dazzling crown of snow on a hill, appear to be 

 near to us ; looked at from the top of a high mountain, the silver glancing curved course of a 

 river not unfrequently appears as if it were raised from the plane. 



False Estimates of Size and Direction. 1. A line divided by intermediate points appears 

 longer than one not so divided. Hence the heavens do not appear to us as a hollow sphere, 

 but as curved like an ellipse ; and for the last reason the disc of 

 the setting sun is estimated to be larger than the sun when it is in 

 the zenith. 2. If we move a circle slowly to and fro behind a slit, 

 it appears as a horizontal ellipse ; if we move it rapidly, it appears 

 as a vertical ellipse. 3. If a very fine line be drawn obliquely across 

 a vertical thick black line, then the direction of the fine line be- 

 yond the thick one appears to be different from its original 

 direction. 4. Zollner's Lines. Draw three parallel horizontal 

 lines 1 centimetre apart, and through the upper and lower ones 

 draw short oblique parallel lines in the direction from above and 

 the left to below and the right ; through the middle line draw 

 similar oblique lines, but in the opposite direction, then the three 

 horizontal- lines no longer appear to be parallel. [Fig. 575 shows 

 a modification of this. The lines are actually parallel, although Fig. 575. 



some of them appear to converge and others to diverge.] If we Zollner's Lines, 



look in a dark room at a bright vertical line, and then bend the 

 head towards the shoulder, the line appears to be bent in the opposite direction (Aubert). 



404. PROTECTIVE ORGANS OF THE EYE. I. The eyelids are represented in section in 

 fig. 576. The tarsus is in reality not a cartilage, but merely a rigid plate of connective-tissue, 

 in which the Meibomian glands are imbedded ; acinous sebaceous glands moisten the edges of 

 the eyelids with fatty matter. At the basal margin of the tarsus, especially of the upper one, 

 close to the reflection of the conjunctiva, open the acino-tubular glands of Krause. The con- 

 junctiva covers the anterior surface of the bulb as far as the margin of the cornea, over 

 which the epithelium alone is continued. On the posterior surface of the eyelid, the conjunctiva 

 is partly provided with papillae. It is covered by stratified prismatic epithelium. Coiled glands 

 occur in ruminants just outside the margin of the cornea, while outside this, towards the outer 

 angle of the eye in the pig, there are simple glandular sacs. Waldeyer describes modified 

 sweat glands in the tarsal margins in man. Small lymphatic sacs in the conjunctiva are called 

 trachoma glands. Krause found end-bulbs in the conjunctiva bulbi ( 424). The blood-vessels 

 in the conjunctiva communicate with the juice-canals in the cornea and sclerotic (p. 737). 



