296 



fi. On an Optical Illusion giving the idea of an Inversion of 

 Perspective in viewing objects through a Telescope. By 

 Professor Forbes. 



" It has been noticed as a curious fact, first, it is believed, by Mr 

 Whitwell, that all parallelograms seen obliquely appear distorted 

 when magnified by a telescope. The effect is to render the space 

 between the upper and lower parallel lines apparently greater as 

 the distance increases, — thus annihilating the usual convergence 

 to a vanishing point, or even reproducing it in an opposite direc- 

 tion. It is hardly necessary to add, that this Inversion of com- 

 mon perspective is only apparent; a fact conclusively established 

 by measuring the angles subtended by the two extremities of a 

 signboard or a row of windows, when the angle evidently dimi- 

 nishes with distance. 



" The effect of magnifying power in apparently distorting ob- 

 jects, may, on the author's view of the subject, be thus concisely 

 stated. The vanishing point of horizontal lines drawn in the 

 same plane is determined wholly irrespective of the distance be- 

 tween them, or the size of the object to be represented. Farther, 

 no movement of the plane nearer to, or farther from, the specta- 

 tor, produces any variation in the position of the vanishing point 

 in the perspective projection. But a telescope which magnifies 

 an object, a signboard, for instance, magnifies the distance of the 

 vanishing point from the point of sight in the same proportion. 

 Hence the perspective is not true for the plane in which the object 

 actually lies ; and if the eye has taken cognizance of the position 

 of the plane by previous inspection, it is easy to shew that it must 

 infer the lines not to be parallel, but diverging from the nearer to 

 the farther part of the parallelogram. On the other hand, if the 

 plane of the object be not previously ascertained, it may appear 

 still to be a parallelogram, but situated in a plane more nearly 

 perpendicular to the line joining the observer's eye and the object 

 than the true plane. Both these effects are really observed." 



3. The following African Shells, collected by J. 0. M' William, 

 Esq., Surgeon R. N., were presented by that gentleman, 

 to the Royal Society, through Dr Traill, who made a few 

 remarks on each, especially the rarest, Nos. 3, 4, 5, 7, 

 and 11. 



1. Achatina Perdix — Accra, Gold Coast. 



2. Achatina Scabra — Isle of Princes. 



3. Achatina Reversa Purpurea — Isle of Princes. 



