SIGHT. 565 



Dr. Magendie has found the matter of the tarsal or Meibomian 

 glands to be not sebaceous but albuminous, and soluble in the 

 tears : hence we discover why, during sleep, it accumulates on 

 the tarsi, because its solvent, the tears, are not sufficiently 

 abundant to remove it. 



" Thus much it was necessary to premise upon the structure 

 of the organ of vision. We now come to the function of the 

 organ, to the explanation of vision." 



Sir D. Brewster, the best authority upon this subject, gives 

 the following data respecting the dimensions, motions, and re- 

 fractions of the eye. The total length of the axis of the eye 

 is about 0*91 inch ; the principal focal distance of the crystal- 

 line lens is 1 *73 ; and the range of the moving eyeball, which 

 determines the field of distinct vision, is 110. The eye be- 

 ing stationary, its field of vision is 120 in the vertical plane, 

 50 being above the horizontal line and 70 below it. In the 

 horizontal plane its field is 150, having 60 inwards and 90 

 outwards. 



The refractive powers of the different humours of the eye are 

 as follows ; the ray of light being incident upon them from air : 



Aqueous Crystalline Lens. Vitreous 



humour. Surface. Centre. Mean. humour. 



1-336 1-3767 1-3990 1-3839 L3394. 



The refractive index of the aqueous humour, it will be ob- 

 served, is almost precisely that of pure water ; and the indices of 

 the other media do not greatly differ from it. 



As the rays refracted by the aqueous humour pass into the 

 crystalline, and those from the crystalline into the vitreous hu- 

 mour, the indices of refraction of each separating surface of these 

 humours will be 



From aqueous humour to outer coat of the crystalline - - 1-0466 



From ditto to crystalline, the mean index being used - - 1 -0353 



From vitreous to crystalline outer coat - - - 1 -0445 



From ditto to ditto, the mean index being used - - 1 -0332 



" Rays of light, falling upon the cornea at an angle more acute 



p p 4f 



