Prof. Forbes on an apparent Inversion of Perspective. 507 
figure M B' C' D', when viewed through an erecting tele- 
scope has the inclination of the lines thrown the other way, 
or the surface then resembles the figures A" B" C" D"; at all 
events no one will hesitate to affirm that the letters really the 
most distant seem to be larger than the nearer ones 
That this singular effect is a mere optical illusion I never 
doubted : and I recently ascertained the fact by measuring 
roughly with a micrometer the apparent angles under which 
A" D" and B'' C' are respectively seen ; the former was al- 
ways found to exceed the latter, in other words the telescopic 
image is really convergent in the same direction with the un- 
magnified one, though the imagination in this case gives so 
completely the lie to the senses, that even when persuaded of 
the deception, and with the invariable standard of the micro- 
meter divisions before our eyes, it is impossible to relinquish 
the preconceived idea. 
The cause to which I assign this effect will appear a natural 
one to persons who are aware of the unperceived and mo- 
mentary train of reasoning by which we arrive at conclusions 
apparently almost intuitive. 
In every case in which the illusion now described is ob- 
* It is scarcely necessary to observe that this anomalous appearance is 
wholly independent of the position of the image in the field of view, and 
is therefore independent of the common errors of aberration. 
