1C> Dr. J. Reade on a ?i?\tj Theory of Telescojxs. 



The following conclusive experiment will explain the true 

 theory of the Galilean telescope. Having placed a glass globe 

 filled with water in a wine glass resting on a table, imme- 

 diately opposite the letter T on the window, the letter was 

 distinctly painted by refraction on the convex surface. I new 

 held the convex object-glass already described at about a foot 

 and half distance over this image, and perceived it to be con- 

 siderably magnified, and surrounded with confused colours. I 

 now interposed the concave eye-glass at about one inch before 

 the image, and perceived the image to be diminished to nearly 

 half its size; but the colours at the margin vanished, and it 

 became, although smaller, much more distinct and black. 

 This image could be either increased or diminished by 

 approaching or distancing the object-glass. I now pasted a 

 piece of white paper on the globe in the vicinity of the re- 

 flected image, and could perceive that the rays which went to 

 form the inverted virtual image, as it has been named, had no- 

 thing to do with the erect image formed on the convex side 

 of the globe by reflection. From this experiment, and nu- 

 merous others, I would infer that in the Galilean tele- 

 scope the eye-glass diminishes the image formed on the pupil 

 by the object-glass; so that a small well-defined image is 

 painted close to the pupil, which standing in place of the 

 object, and magnified by the object-glass to the proper 

 standard of distinct vision, I now made an assistant seat 

 himself opposite this letter on the window; and on holding the 

 glasses in the same manner before his pupil, 1 perceived si- 

 milar effects ; and when the white paper was pasted on half 

 the eye-glass, on his looking at the letter, I perceived the 

 erect image in the pupil, and the inverted image painted on 

 the paper at the eye-glass. I now instead of the white paper 

 put on a piece of white cloth, so as to hinder those rays from 

 going at all to his eye ; yet he perceived the object through 

 the telescope. After this experiment, can any person contend 

 that this virtual image is the mean of vision through the 

 telescope ? The field of view in the Galilean telescope depends 

 on the distance of the object-glass, for the size of the image is 

 inversely as the distance. 



Thus having shown by numerous experiments that these 

 gross refracted rays, with all their twistings and turnings, have 

 nothing to do with the phaenomenon of vision either naked or 

 armed with the telescope, further than by throwing a quantity 

 of light into the pupil, I shall now give a figure. 



a, the letter T pasted on the window, sends an image to 

 flie object-glass B C, where \\yo more images are formed in 



each 



