Variation of the Optical Deception of Le Cat. 89 



aerial perspective, provided the difference in the distances of 

 the nearer and the remoter end is considerable, and the air 

 sufficiently hazy. 



The field, however, may be so situated, and have such an in- 

 clination, that, when seen through the telescope, it appears like 

 a pei-pendicular or vertical wall of earth. This phenomenon 

 we have often seen in directing a telescope to a field above 

 Melrose 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 a vertical plain rising in front of him. This deception 

 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. I have not yet observed the effect 

 produced when it is in the act of being ploughed. It is very 

 probable that the impossibility of ploughing a vertical plain 

 may remove the deception, upon the principles which we have 

 explained in a subsequent article. 



No. VIII. Variation of the Optical Deception of Le Cat. 



M. Le Cat has described in his Traite des Sens, p. 298, 

 a curious optical deception, in which an erect object placed 

 near a hole in a card next the eye, will appear to be on the 

 other side, and also inverted and magnified. Let CD, Plate 

 I, Fig. 9 be a card perforated with a small hole, E a white 

 wall or window, D the eye of the observer, and d the head of 

 a pin held near the eye, and also near the hole in the card. 

 Under these circumstances the pin d will be seen at F invert- 

 ed and magnified. The reason of this is, as M. Le Cat has 

 observed, that the eye in this case sees only the shadow of the 

 pin on the retina, and since the light which is stopped by the 

 upper part of the pin or its head comes from the lower part 

 of the white wall or window F, and that which is stopped by 



