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LXXVII. On an apparent Inversion of Perspective in viewing 
Objects with a Telescope, By James D. Forbes, Bsq., 
F.R.S., Professor of Natural Philosophy in the University 
of Edinburgh,^ 
I N October last, Sir John Robison directed my attention 
to a curious anomaly in the apparent perspective of ob- 
jects seen through a telescope which had been first mentioned 
to him by Mr. Whitwell. 
It consists in a complete seeming inversion of the true in- 
clination of two horizontal lines towards a vanishing point 
when seen through an ordinary telescope. The top and bot- 
tom lines of a row of windows, for instance, viewed obliquely, 
seem, within the limit of the field of view of the telescope to 
converge to a point on the opposite hand of the spectator 
from that indicated by the common rules of perspective, and 
by the experience of the naked eye. There is no better ex- 
emplification of the fact than by viewing the figure of a com- 
mon sign-board, not far from the eye and considerably fore- 
shortened, with a common pocket telescope. The letters 
appear gradually to increase from the nearer towards the 
more remote part of the inscription. 
That the appearance is such as we describe no one will 
readily admit who does not make the experiment for himself ; 
but once made, the fact appears so certain as to create sur- 
prise, that it does not always strike us, and that it has not 
(so far as I am aware) been mentioned in books on such sub- 
jects. 
The first time I saw the anomalous appearance in com- 
pany with Sir John Robison and Mr. W. A. Cadell, the 
explanation which I am about to state occurred to me as 
the true one. Not being particularly conversant with the 
subject of perspective, I contented myself with stating my 
opinion in writing at the time, and should probably have 
never recurred to the subject, had I not been lately requested 
to examine an ingenious paper, in which the reality of this 
distortion was admitted, and an attempt made to account for 
it by tracing the path of the rays through the telescopic lenses. 
Conceiving these investigations to be unsatisfactory, I made 
one or two simple experiments, which satisfied me completely 
of the accuracy of the view which I had previously taken of 
the matter, and which I now proceed to state. 
The fact to be accounted for is, that a parallelogram, 
A B C D, or a v»^ord composed of letters of equal height, 
which by common perspective assumes to the naked eye the 
Communicated by the Author. 
