in vie^ming Objects mth a Telescope, 509 
plane, or in any plane parallel to that, have a single vanishing 
point ; but the act of magnifying increases the distance of the 
vanishing point in the same proportion as it does the ap- 
parent dimensions of the object; consequently the magnified 
object is not seen in true perspective for its own plane. 
Edinburgh, March 13, 1840. JaMES D. Forbes. 
Postscript. — A casual circumstance brought to my recol- 
lection a few days ago an optical illusion mentioned (if indeed 
it was not shown) to me some years since by Sir David Brew- 
ster, the nature of which I could not perfectly recollect. 
Having applied to Sir David Brewster, he obligingly referred 
me to the Edinburgh Journal of Science for a notice of it, 
when it proved, as I expected, to be referable to the principle 
I have just been applying. 
A field,” says Sir David, may be so situated, that ” 
(from the perspective of the furrows or drills upon its sur- 
face) “ when seen through the telescope it appears like a per- 
pendicular or vertical voall of earth. This phaenomenon we 
have often seen in directing a telescope to a field above Mel- 
rose Abbey on the northern acclivity of the north-west 
Eildon Hill. This field is capable of being ploughed in the 
direction of its greatest declivity ; but when it is viewed 
through a telescope, the slope is such that the furrows do not 
appear to converge^ and the eye cannot readily perceive any 
difference between the breadth of the furrows at the remote 
end of the field and their breadth at the near end. The ob- 
server therefore immediately concludes that the field must be 
nearly in a vertical plane rising in front of him. This de- 
ception is a very remarkable one, and produces a singular 
effect on the mind when the field is covered with a crop, and 
when crows, &c. light upon it.” 
A more perfect illustration of the second form of the optical 
illusion which I have described could not be desired. Every 
one knows how imperfectly the eye estimates the acclivity of 
a plane in full view. The parallelism of the ridges is tacitly 
assumed, and as their apparent convergence diminishes ex- 
actly in proportion as the magnifying power of the telescope 
increases, the mind is forced to the conclusion that the plane 
is more nearly perpendicular to the line joining the eye and 
any point of it, than it really is. 
Flence it appears that Sir David Brewster noticed and 
published fourteen years ago one case of the curious obser- 
vation of Mr. Whitwell. 
March 17, 1840. 
* First; Series, iii, p, 88. 
