Vision under Water. 183 



mind of certain impressions made upon the retina, and communicat- 

 ed through the medium of the optic nerve to the encephalon ; a 

 sound condition of each and all of these parts which may be con- 

 sidered as the media of communication, so far as one sense is con- 

 cerned, between the external world and the mind, is indispensable 

 for perfect vision. Light may fall upon the retina, and the images 

 of objects may be there depicted ; but should the optic nerve be un- 

 sound, or certain portions of the brain be disorganised, no responsive 

 image is called up before the mind ; the eye may gaze upon the 

 noonday sun, but all is dark within. 



The natural stimulus of the retina is the luminous rays ; the ap- 

 preciation of light and colour its active condition ; and its state of 

 repose suggests the appearance of darkness ; but besides light, any 

 other excitement applied to the retina or optic nerve gives rise 

 to the same result, the production of luminous appearances. Pres- 

 sure upon the eyeball, the electric current, or vascular congestion, 

 all excite this special phenomenon. Occasionally, too, irritation of 

 the brain has the same effect ; and many are the waves and corrus- 

 cations, the fiery clouds and flaming spectra, which haunt the amau- 

 rotic when certain morbid complications exist. The phantasms of 

 fever, and the illusions of the dying, are to be placed in the same 

 category with the above. — (Todd's Cyclopedia.') 



8. Vision under Water. — Vision under water is attended with 

 some curious consequences, the result of what is termed " internal 

 reflection. An eye placed under perfectly still water, as for instance, 

 the eye of a diver, will see external objects only through a circular 

 aperture (as it were) of 96° 55' 20" in diameter overhead. But all 

 objects down to the horizon will be visible in this space ; those near 

 the horizon being much distorted and contracted in dimensions, espe- 

 cially in height. Beyond the limits of this circle will be seen the 

 bottom of the water, and all subaqueous objects reflected and as vi- 

 vidly depicted as by direct vision ; and, in addition, the circular 

 space above mentioned, will appear surrounded with a rainbow of 

 faint but delicate colours. In the eyes of fishes, the humours being 

 nearly of the refractive density of the medium in which they live, 

 the action of bringing the rays to a focus on the retina is almost en- 

 tirely performed by the crystalline lens, which is nearly spherical, 

 and of small radius in comparison with the whole diameter of the 

 eye; there is also a very great increase of density towards the 

 centre, whereby spherical aberration is obviated, the corneal refrac- 

 tion having little influence. — flbid.) 



9. Colours most frequently Hit during Battle. — It would appear, 

 from numerous observations, that soldiers are hit during battle ac- 

 cording to the colour of their dress, in the following order : — Bed is 

 the most fatal colour ; the least fatal, Austrian grey. The propor- 

 tions are, — Red 12 ; Rifle green 7 ; Brown 6 ; Austrian bluish-grey 5. 



