ikct. v.] DISSERTATION SECOND. 183 



derstood. Maurolycus had nearly discovered the secret, 

 and it was but a thin, though, to him, an impenetrable veil, 

 which still concealed one important part of the truth. 

 This veil was drawn aside by the Neapolitan philosopher ; 

 but the complete discovery of the truth was left to Kepler, 

 who, to the glory of finding out the true laws of the plane- 

 tary system, added that of first analyzing the whole scheme 

 of nature in the structure of the eye. He perceived the 

 exact resemblance of this organ to the dark chamber, the 

 rays entering the pupil being collected by the crystalline 

 lens, and the other humours of the eye, into foci, which 

 paint on the retina the inverted images of external ob- 

 jects. By another step of the process, to which our ana- 

 lysis can never be expected to extend, the mind perceives 

 the images thus formed, and refers them at the same time 

 to things without. 



It seemed a great difficulty, that, though the images be 

 inverted, the objects are seen erect ; but when it is con- 

 sidered that each point in the object is seen in the direc- 

 tion of the line, in which the light passes from it to the re- 

 tina, through the centre of the eye, it will appear, that the 

 upright position of the object is a necessary consequence 

 of this arrangement. 



Kepler's discovery is explained in his Paralipomena in 

 Vitellionem, ' (Remarks on the Opticks of Vitello,) a work 

 of great genius, abounding with new and enlarged views, 

 though mixed occasionally with some unsound and visiona- 

 ry speculations. This book appeared in 1604. In the 

 next article we shall have occasion to return to the con- 

 sideration of other parts of Kepler's optical discoveries* 



1 Caput 5. de IVJodo Visionis. 



