Chap. 8.] Re/rafting and Galilean ' fekfcope. 253 



parallel to the lines drawn from each point through 

 the center of the lens. Thus the firft image of QJ 3 

 (fig. 44.) is qp y the fecond K I. The apparent mag- 

 nitude of the object is not changed by thefe glaJTes, 

 and depends, as before, on the focal lengths of the ob- 

 ject-glafs and lens neareft to it. The brilliancy of the 

 object, however, will be diminifhed, fince feveral rays 

 will be loft in their paflfage through the two additional 

 glaffes. 



An inflrument made with glafles combined together 

 in this manner, and inferted in a tube, is called a 

 refracting telefcope ; the latter word implying, ac- 

 cording to its fignification in the Greek language, the 

 property of feeing objects at a diftance. In placing 

 the glafles in this tube, care muft be taken that the 

 axes of the lenfes coincide, or, as .it is evident from 

 cur principles, indiftinct vifion only will be pro- 

 duced. 



Inliead of the two additional glafles, Galileo chang- 

 ed the convex eye-glafs into a concave one, by which, 

 as to the magnifying power, the fame effect was pro- 

 duced. Thus (fig. 45.) qp being the image of QJP, 

 by placing a double concave lens between this image 

 and the object-glafs, the rays converging to p ap- 

 pear to diverge from R, and I R, the image feen 

 by the eye at Q^, is made erect. In this cafe, the 

 nearer the eye is placed to the glafs, and the greater 

 the pupil, the more the object will be feen. 



A microfcope, or an inflrument formed to inipect 

 minute objects, is conftructed exactly on the fame 

 principles. A globe of glafs, or double convex lens, 

 will, from what has been faid, anfwer the purpofe ; 

 or, a minute object, feen through two convex lenfes, 

 will be magnified in the proportion of the focal length 

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