./. LeConte on f 



all objects or images in the tields oi view seem to move while 

 the visual lines seem stationarv. As the \ isual line of the 

 right eye moves to the left all the iniaLies in its field seem to 

 move to the right, and as the visual line ot the left eve moves 

 to the right all its images move to tlie left. Thus as the visual 



these objects move forward, and sueeessively take jiositioii 

 directly infrmit] so that under all eireunistanees the t\v.. visual 

 lines combine to form a median visual line passintz- throu^di the 



C"nt of sight and onward to InliHitv. In ease the imai^vs thus 

 >ught together are sinular, as in uiv [irevious exi»efiiuents. 

 they combine, and are seen single at the point of sight : hut 

 otherwise they maintain their relative distances. This is espe- 

 cially true when we look at continuous lines on a plane parallel 

 to the visual plane, as in the experiments detailed above. The 

 fields rotate toward each other and images come together on 

 the median visual line, but maintain their relative distances 

 from the observer. 



I might multiply experiments of this kind without number, 

 but I wish now to show into what singular confusion some of 

 the most careful observers have fallen by adoj.ting tlie usual 

 mode of representation. I will give first an exjteriinent of 

 Clapar^de which, according to him, is only a eontii-niation of 

 what had been previously observed in the last century Ity 

 Robert Smith, again in 1818 by Yieth, and again in 1838 by 

 Wheatstone. It is therefore well fortified by great names. I 

 translate :*— " Take a compass widely opened (fig. 24), apply its 

 head to the root of the nose, holding the branches m tlie plane 

 of vision. The screen S is so disposed that the left eye L sees 

 only the branch a, and the right eye R only the brandi L If 

 now we press the branches until the points are lirouLdii on the 

 optic axes, (the point of sight being at A) as indieate<l in the 

 figure (fig. 24); we will perceive a closed rnu</K>^^ betw.-en the 

 images ss s's' of the screen (fig. 25), and this .-mnpass stretches 

 itself even to the point of view A.'" Now 1 hav.^ re|.eatedly 

 tried this experiment and I find the visual result eutirelv differ- 

 ent. This result is shown bv my mode ot representation in fig. 

 26. It is two half compasses' united at the point and ,ep,,raied at 

 the head, hy the whole comn\on field of vkv. It cannot be other- 

 wise, since we must see two noses bounding the common field 

 of view on each side, and the head of the compass resting on 

 the nose must be similarly doubled and the images smiilarly 

 separated. As to length, the compass may be imagined of mxy 

 extent short of the point of sight A. If the compass is placed 

 ^actly in the plane of vision, the experiment succeeds equally 

 * Bib. Univ. Archives des Scien.. 11. Tome .3. p. 254. 



