424 



Mr. J. L. Tupper on an Optical Illusion. 



must converge to a point, or there would be no distinct picture 

 of them on the retina. An object, to become visible, must then 

 be placed further than the principal focus of the eye ; if an object 

 be placed between that focus and the eye, it becomes directly in- 

 visible {p. ex. the e} T elashes); but that object may intercept the 

 rays emitted by another visible object situated further — in a 

 word, it may act as a screen. These particulars understood, let 

 A (fig. 4) be the eye, M N a white illuminated surface, P Q the 



Fig. 4. 



bent card. It is easy to understand that of all the white rays 

 emitted by the surface M N, those alone that are situated on the 

 circle C D can enter into the eye, crossing at the aperture E. 

 If, now, an object be placed on the cone E C D, this object, act- 

 ing as screen, will intercept some rays of the circle CD, and the 

 eye will perceive in black the image of this object H on the 

 circle. The same must take place when the object is in G, pro- 

 vided it be reversed ; then, as all the rays cross at E, the object 

 G will intercept the same rays as H (supposed E G — E H), with 

 that difference, that at G the object, being directly invisible, can 

 only act as a screen, while at H it could at the same time act 

 as a screen for the surface M N and be visible, if it is in the con- 

 ditions of distance above mentioned. This will explain, then, 

 why the object G is projected in M N in an inverse position. — 

 Michael." 



Here the writer explains the erect appearance of the inverted 

 pin G by identifying it with the erect appearance of the erect pin 

 H, as seen by crossing rays — quite forgetting that, if objects or- 

 dinarily appear in their true position, a special arrangement 

 which inverts them before they reach the eye will make them 

 appear reversed, that H therefore ought to appear reversed; 

 and that " why it does nGt " demands explanation as much as 

 " why G does." He has, by employing two cones of rays, 

 brought the pin on to the lens in an inverted position, and yet 

 expects that we should see it erect, in defiance of hourly expe- 

 rience, that just as objects stand upon the lens so they are seen. 



