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LXXVII. On a7i apparent Inversion of Perspective in viewing 

 Objects with a Telescope. By James D. Forbes, Esq., 

 F.JR.S., Professor of Natural Philosophy iii the University 

 of Edinhurgh."^ 



IN 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 word composed of letters of equal height, 

 which by common perspective assumes to the naked eye the 



* Communicated by the Author. 



