THE SENSES. 203 



I 



rapidity with which light travels, and from not having any other 

 sense that can give us information more quickly. There is always 

 an interval, however, and in the case of the distant heavenly bodies 

 this has been calculated. We have said sound travels at the rate 

 of between twelve and thirteen miles in a minute, but light passes 

 through 195,000 miles in the sixtieth part of the same time. 



As the eye is strictly an optical instrument, we must state that it 

 is a law of optics that the rays of light, while passing through the 

 same medium, proceed in straight lines, but that they are turned 

 out of their course when they pass from a less into a more dense 

 medium. They are then said to be refracted. This takes place 

 when the rays of light pass from air into water, and it is by virtue 

 of the same law that a common magnifying or double convex glass 

 collects the sun's rays into a focus or point. 



. The eye has various appendages, which require some explana- 

 tion. The first to be noticed are the eyelids. These are composed 

 chiefly of a gristly substance placed under the skin that accurately 

 fits the ball of the eye, and which is lined internally by a thin mem- 

 brane called the conjunctiva, that turns over on the globe of 

 the eye, and keeps it in its socket. Attached to the eyelids are 

 the eyelashes, which protect the eye from too great a glare 

 of light, from particles of dust, &c. Persons without eyelashes 

 have always tender eyes. The chief purposes served by the eye- 

 lids are, 1st,. to protect from external injury, and to exclude the 

 light when they are closed ; and, 2dly, to distribute equally over the 

 eyeball the fluid which moistens it. This fluid is usually carried 

 off as quickly as it is formed ; but when the eye is irritated, or the 

 mind affected by various emotions, it is then secreted in such quan- 

 tity as to run over the eyelids in the form of tears. - The source of 

 this fluid is a gland, named the lachrymal gland, situated above the 

 outer angle of the eye. Tears, there secreted, pass downwards to 

 the eye, whence they flow, through two small holes (puncta lachry- 

 malia) near the inner angle of the eyelids, into a small receptacle 

 called the lachrymal sac, placed immediately behind the inner 

 angle, and from which there is a communication to the nostrils by 

 what is called the nasal duct. This is the reason why, when tears 

 are copious, a necessity for blowing the nose is felt. When the 

 nasal duct is obstructed, as often happens, the nostril on that side 

 is dry, and the tears run over the eyelids. The puncta lachrymalia 

 may easily be seen by everting the eyelids, and looking at their 

 inner angle ; and the opening of the nasal duct may be seen by look- 



