Dr. J. Reade on a new Theory of Telescopes. 23 



PQR will be formed upon the retina of a common eye. 



Also, the angle under which the object Q P is seen through 

 the telescope is equal to the angle q Ep; and the angle under 

 which it is seen by the naked eye at L is QL P, which is equal 

 to q Lp. Therefore the visual angle in the former case : the 

 visual angle in the latter : : L q : E q. " From this reasoning 

 Mr. Wood infers the corollary that the magnifying power of 

 the Galilean telescope is measured by — . 



Before I enter on a mathematical examination, I shall beo- 

 leave to call the attention of my readers to what I conceive 

 to be a metaphysical absurdity attending this theory : I shall 

 quote a passage from Dr. Young, an able mathematician and 

 experimental philosopher ; who says in his Lectures on Na- 

 tural Philosophy, p. 428, " In the Galilean telescope or opera- 

 glass, a concave eye-glass is placed so near the object-glass 

 that the first image would be formed beyond it and near its 

 principal focus, and the second image formed by the eye-glass, 

 which is the vertical image viewed by the eye, beino- on the 

 opposite side of the centre, is inverted with respect to^the first 

 image, and erect with respect to the object." And in page 

 427, he continues, that " in almost all telescopes and com- 

 pound microscopes the image formed by one lens or mirror 

 stands in the place of a new object for another. The opera- 

 tion of such instruments may be illustrated (say Dr. Young 

 and other authors) by placing a screen of fine gauze in the 

 place of the image, which receives enough of light to make the 

 image visible in all directions, and yet transmits enough to 

 form the subsequent image." 



Here let us pause, and mark the gross metaphysical ab- 

 surdity, in supposing that an inverted image painted on the 

 retina could enable the observer to see both an erect object 

 through the telescope, and an inverted object or image at the 

 piece of gauze. Again : Dr. Young supposes that this in- 

 verted image at the gauze transmits enough of rays to form 

 the subsequent image on the retina. Now it is evident this 

 retinal image would be erect in respect to the image on the 



gauze, 



