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HISTORICAL VIEW OF THE

the moon’s surface is still illuminated by rays refractedin their passage through the earth's atmosphere.

The constitution of the eye, if there be anything inmathematical science which comes home with a directpersonal concern to every one, is that which is of themost intense interest. We have observed the stepswhich had been made towards the investigation of themode in which vision takes place, by Maurolycus andBaptists Porta: the former nearly discovered the secret;and it was but a thin, though to him impenetrable veil,which still concealed one essential part of the truth fromthe Neapolitan philosopher. This veil was drawn aside,and the complete discovery of the truth effected byKepler, who, to the glory of finding out the laws ofthe planetary system, added that of first analysing thewhole scheme of nature in the structure of the eye. Heperceived the exact resemblance of this organ to thecamera obscura; of the rays entering the pupil, theseparate small pencils from all points in the object, crossat the centre of the crystalline lens, and are each broughtto their respective foci in the vitreous humour, whoserefractive power, compared with that of the lens, isprecisely adjusted so that the focal distance shall coin-cide with the utmost exactness with the position of theretina, when the succession of those luminous pointspaint an inverted picture of external objects. But hereagain, by another law, in whatever direction a ray fallson the retina, we, by instinctive judgment, refer it toan object in that direction produced ; hence the raycoming downwards, or falling on the lower part of theretina, is referred by us upwards, or to the upper partof the object; or, in other words, we see objects erect.When we have traced the process to the picture on theretina, we have arrived at the limit to which optics cancarry us. Physiology may yet show whether any par-ticular action of the nerve is connected with vision ; butthe boundary between the physical impression and themental perception will probably never be penetrated.

Kepler’s affairs in the meanwhile continued in the