258 FERGUSON'S LECTURES. 



LECT. picture a b upon the retina, whereon it is magnified 

 ^^^ under the angle b Fa. 



But this telescope has one inconvenience which ren- 

 ders it unfit for most purposes, which is, that the pencils 

 of rays being made to diverge by passing through the 

 concave glass n o, very few of them can enter the pupil 

 of the eye ; and therefore 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 

 all 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 teles- 

 cope 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 con- 

 vex eye-glass, as g h in place of the concave one ; and 



fixing it so in the tube, that its focus may be coincident 

 with the focus of the object-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 cd, will meet 

 in its focus, and form the inverted image m Ep : and 

 as the image is formed in the focus of the eye-glass # A, 

 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 ge h ; and 

 being at E, it will appear magnified, so as to fill the 

 whole space Cmep D. 



