50S Prof. Forbes on an ap-parent Inversion of Perspective^ 



served, the spectator has previously determined by his eye the 

 real position of the plane of the object towards which he di- 

 rects his lelecope : and when he views that object with a mag- 

 nifying power of two, he believes himself to be looking at an 

 object twice as large in the same plane as before; or else 

 (what comes to the same thing) the same object as before 

 brought twice as near to him, but moved parallel to itself. 

 In either of these cases, the vanishing point ought to remain 

 exactly the same for the enlarged as for the original object. 

 If a h c dhe^ board 4 feet long and 1 foot high, the eye expects 

 to see through the telescope magnifying twice, a figure similar 

 to that which a board 8 feet long and 2 high would present 

 in the same situation, thai is, a figure a' b' c' d of which the 

 upper and lower lines converge to the same point V as before; 

 ut the eye really sees through the instrument a merely mag- 

 nified image of a b c d, namely a' /3 c d, in which a' /3 is 



Y 



parallel to a b, and consequendy the vanishing point V is 

 thrown twice as far off. What must the mind, reasoning 

 through the information lent by the eye, infer respecting this 

 enlarged object? One of two things. Either that the sign- 

 board so seen is really not a parallelogram, but has its further 

 extremity b' c' higher than the nearer one ;. or else, that the 

 board is a true parallelogram, but that the plane in which it 

 lies is more nearly perpendicular to the line joining the eye 

 and the object, a plane in short which will give to horizontal 

 lines a vanishing point as far beyond V as that is from d. 



The former is the case when we look at an object to which 

 we direct a telescope after having mentally formed an estimate 

 of its position ; the latter, or an erroneous estimate of a plane 

 of the object, occurs when a person looks suddenly through 

 a telescope previously pointed in an unknown direction. 



I am not sufficiently conversant with works on perspective 

 to be aware whether such a circumstance has before been no- 

 ticed, but it was new to those whom I have had occasion to 

 consult. 



At all events it is very singular that it should have remained 

 so long generally unknown that all objects (generally speak- 

 ing) are seen through a telescope in J atse per sp)ective. 



The general principle may be thus stated in a single sen- 

 tence. By common perspective, all parallel lines in a single 



