OPTICS. QQQ 



the field of view is but very small, as is evident by 

 the figure. For none of the pencils, which flow 

 either from the top or bottom of the object A B, 

 can enter the pupil of the eye at C, but are stopped 

 by falling upon the iris above and below the pupil; 

 and, therefore, only the middle part of the object 

 can be seen when the telescope lies directly towards 

 it, by means of those rays which proceed from the 

 middle of the object. So that to see the whole of 

 it, the telescope must be moved upwards and 

 downwards, unless the object be very remote; and 

 then it is never seen distinctly. 



This inconvenience is remedied by substituting 

 a convex eye-glass, as g h (Fig. 2.), in place of the 

 concave one; and fixing it so in the tube, that its 

 focus may be coincident with the focus of the ob- 

 ject-glass c d; as at E. For then the rays of the 

 pencils flowing from the object A B, and passing 

 through the object-glass c d, will meet in its focus, 

 and form the inverted image m E p ; and as the 

 image is formed in the focus of the eye-glass g h, 

 the rays of each pencil will be parallel, after passing 

 through that glass; but the pencils themselves will 

 cross in its focus, on the other side, as at e\- and 

 the pupil of the eye being in this focus, the image 

 will be viewed through the glass, under the angle 

 g e h; and being at E, it will appear magnified, so 

 as to fill the whole space C m e p D. 



But, as this telescope inverts the image with re- 

 spect to the object, and is only fit for viewing the 

 heavenly bodies, in which we regard not their 

 position, because, on account of their being round, 

 they do not appear inverted. 



The magnifying power ot this telescope is, as 

 the focal distance of the object-glass to the focal 

 distance of the eye-glass. Therefore, if the former 



