THE SENSES 555 



the nostril ; but it is essential that the punctum should be in con- 

 tact with the globe, or else the tears run over the side of the face. 

 The use of the tears is to keep the conjunctiva moist and polished, 

 and to wash away foreign bodies. 



The Eyelashes of the horse are peculiar. Those on the lower 

 lid are very few and fine, whilst on the upper lid they are 

 abundant, and exist not as a single but as a double row ; the 

 rows cross each other like a trellis-work, but without interlacing ; 

 these eyelashes are very long and strong (Fig. 171). A few pro- 

 tective hairs grow from the brow and below the lower eyelid ; 

 in some horses they are 4 or 5 inches in length. They appear to 

 be in connection with nerve terminations, and their delicacy to 

 the sense of touch is remarkable. The function of these hairs is 

 protective ; they give the eyes warning of danger. 



Fig. 172. Fig. 173. 



Figures illustrating the Action of Lenses upon Ravs of Light 

 passing through them. 



Fig. 172 — biconvex leas : O, optical centre ; mm, chief or principal axis; nn, 

 secondary axis. 



Physiological Optics — Passage of Light through Lenses. — All rays of 

 light are diverging, but so slight is the divergence of the rays from 

 distant objects, that for the purposes of the eye they are practically 

 regarded as parallel. All rays proceeding from an object situated at 

 from 6 metres (20 feet) to infinity from the front of the eye are con- 

 sidered as parallel rays ; all rays within 20 feet from the cornea are 

 diverging rays. Obviously the nearer the object to the cornea the 

 greater the divergence, so that there is more divergence in the rays 

 proceeding from a body 1 foot from the eye than in one 10 feet from 

 the eye ; conversely, the farther the object is from the eye the less 

 divergent the rays until that point is reached beyond 20 feet at 

 which the rays may be regarded as parallel. 



A convex lens has two curved surfaces, and a line drawn through 

 the centre of these two surfaces is known as the principal axis of 

 the lens (Fig. 172, mm). The essential idea of a double convex 

 lens is that it is thicker at the centre than at the edges. Situated 

 on the principal axis of a biconvex lens at a point in its interior is 

 the optical centre (Fig. 173, O) ; any other straight line passing through 

 the optical centre is termed a secondary axis (Fig. 172, nn). 



When parallel rays of light (Fig. 173, a) pass through a convex 

 lens they are refracted and brought to a point / on the opposite side 

 of the lens, known as the principal focus ; the only rays not refracted 



