240 ANATOMY FOR NUKSES. [CHAP. XIX. 



and the pupil of the eye dilated, and it is on this account that 

 the eye rests for an indefinite time upon remote objects without 

 fatigue. 



The eyeball is often compared to a photographer's camera. 

 It is essentially a hollow spherical box filled with fluids, hav- 

 ing its interior surface darkened by pigment, and containing a 

 system of lenses by means of which images can be formed, and 

 a screen upon which they can be received. In front is a cur- 

 tain or diaphragm (the iris), with a variable central aperture 

 (the pupil) to regulate the amount of light admitted. 



The colour of light is considered to be analogous to the pitch of 

 sound. As the latter is determined by the number of vibrations 

 of the atmosphere which strike the ear in a second, so the former 

 depends on the number of the waves of ether which strike the 

 retina in a second. The lowest note of an ordinary musical scale 

 has, as we have already remarked, sixteen vibrations per second ; 

 the highest, 38,000 per second. The number of ether-waves 

 which strike the retina in a second to produce the sensation of 

 red (which lies at the bottom, so to speak, of the colour-scale) 

 is estimated at 474,439,680,000,000. The number required to 

 cause the sensation of violet, which lies at the other extreme of 

 our colour-perception, is estimated at 699,000,000,000,000 ! 



The muscles which move the eyeball are the four straight or 

 recti and the two oblique. They have been sufficiently de- 

 scribed on page 60. 



The appendages of the eye are the eyebrows, eyelids and 

 lachrymal glands. 



The eyebrows are composed of two arched eminences of 

 thickened skin, connected with three muscles, which by their 

 action control to a limited extent the amount of light admitted 

 into the eye. The eyebrows are furnished with numerous 

 short, thick hairs, lying obliquely on the surface. 



The eyelids are two folds, projecting from above and below 

 in front of the eye. They are covered externally by the skin 

 and internally by a mucous membrane, the conjunctiva, which 

 is reflected from them over the globe of the eye. They are 

 composed for the most part of connective tissue, which is 

 dense and fibrous under the conjunctiva, where it is known 

 as the tarsus. 



Embedded in the tarsus is a row of elongated sebaceous glands 



