24 Dr. J. Reacle on a new Theory of Telescopes. 



gauze, and therefore the observer should see it inverted : but 

 why see the object erect? I am inclined to hope that Dr. 

 Young will agree with me, not only that this experiment 

 and reasoning involve a metaphysical absurdity, but that 

 they strike at the retinal theory of vision. For to perceive 

 an object both erect and at the same time inverted, by 

 means of one image on the retina, is impossible. But there is 

 no difficulty with respect to the corneal theory of vision ; for 

 on holding a concave and convex glass before the eye of an 

 observer looking at the letter on the window, I distinctly per- 

 ceived a small erect image of the letter on the pupil, and like- 

 wise an inverted image produced by the refracted rays as 

 they are called. 2dly, The eye of the observer is always 

 placed close to the eye-glass. If the magnitude of the object 

 seen through the telescope depended on the visual angle q Ej5, 

 the object should appear larger the further the eye was re- 

 moved from the eye-glass ; but direct experiment teaches that 

 the contrary is the fact. Indeed, the idea of an inverted 

 image floating in the air, invisible, yet visible, and measured 

 by a visual angle beyond the influence of the nerves, which 

 visual angles are measured by the calf as well as the cow, 

 by the idiot as well as the philosopher, is more than we can 

 believe : even the learned Berkeley, late bishop of Cloyne, who 

 advanced many strange things, could not believe in this. The 

 next object is, that these focal and vertical images, from which 

 our ideas are supposed to be derived, are ten or twenty times 

 as large as the area of the pupil itself, and consequently a con- 

 jurer may as well get into a quart bottle as these images through 

 the pupil. This is a strong objection, and I conceive an in- 

 surmountable one, to the present theory of telescopes. Indeed 

 I am surprised it should have escaped notice. How is it 

 possible that the extreme rays coming from a vertical image 

 one inch or more in length and half an inch in breadth, could 

 not only enter the pupil, but a hole the size of a pin's head, such 

 as that used by surveyors? We must either admit, what op- 

 ticians do not allow, that rays converge after passing through 

 the concave eye-glass so as to form a very sharp cone at the 

 pupil, and do away with the magnifying power, illustrated by 

 the angle q E/>, otherwise the cone would be at the wrong 

 end. Having unscrewed the eye-glass of an opera telescope 

 about three inches long, I held it before the window, and, on 

 placing a card at the principal focus, found that focus to be 

 about five inches from the object-glass. The letter T on the 

 window, being ten feet distant when the eye-glass was again 

 screwed on, the focal image became somewhat enlarged, but 

 very indistinct. What I principally wish to impress is the 



fact, 



