22 Dr. J. Reade on a neiv Theory of Telescopes. 



a rule: yet it must be allowed that on a subject of such im- 

 portance to the happiness of mankind, such a method is very 

 unsatisfactory, and often leading to an injury of sight, for two 

 convex or concave spectacle glasses may be very dissimilar in 

 transparency or quality of glass without in the least affecting 

 the focal images ; this may be shown by scratching a lens or 

 soiling the surface without in the least altering those focal 

 images. The following method, arising out of the discovery 

 that the cornea is the true seat of vision, is preferable. Pro- 

 cure two alass irlobes somewhat lamer than the human eye, fill 

 them with pure water, and place them in a case at about one 

 inch from each other to represent the human eyes. The glasses 

 for examination, whether convex or concave, should now be 

 placed in their frames and held in front of these globes, and 

 before the letter T pasted on the window. On comparing 

 the magnified or diminished images of the letter on the con- 

 vex faces of these globes, the practical optician may imme- 

 diately say whether they are similar in their powers, whether 

 they magnify or diminish equally, and likewise whether the 

 images are clear and distinct. In making this experiment, we 

 should take care to have the globes perfectly similar. Here- 

 after I shall resume this subject, and shall now proceed to the 

 theory of telescopes. 



As die Galilean telescope is the most ancient, most simple, 

 and most generally used, I shall begin with it. For a good 

 history of this instrument I must refer to an excellent article 

 in Brewster and Rees's Encyclopaedias, and in a concise man- 

 ner shall give the present theory from Mr. Wood's Elements 

 of Optics, one of the best works I have seen on the subject, 

 and I am informed the one used at Cambridge, perhaps the 

 first mathematical university in Europe. " Galileo's telescope 

 consists of a convex object-glass, and a concave eye-glass, 

 whose axes are in the same line, and whose distance is equal 

 to the distance of their focal lengths. A distant object may 

 be seen distinctly through Galileo's telescope, and the angle 

 which it subtends at the centre of the eye when thus seen, is 

 to the angle which it subtends at the centre of the naked eye 

 as die focal length of the object-glass to the focal length of the 

 eye-glass. 



" Let L and E be the centres of the glasses, PQR a distant 

 object towards which the axis of the telescope is directed ; 

 pqr its images in the principal focus of the glass L, and 

 therefore in the principal focus of the glass E. 



Then since the rays tend to form an image in the principal 

 focus of the concavi- lens E, after refraction at that lens they 

 will be proper for vision, or a distinct image of the object 



PQR 



