OBSERVING AND MANAGING ENGISCOPB. 181 



that thou art not to do. I will try to make thee know the 

 right end of thy instrument from the wrong one ; how 

 to put a fly's eye before the object-glass, and a fool's eye 

 before the eye-piece; with many other things equally 

 curious, important, and interesting ; and if perchance I 

 shall succeed in learning thee how to deal with the 

 instrument under consideration, the management of all 

 others constructed on similar principles must be per- 

 fectly obvious. 



It will be necessary for me to premise somewhat con- 

 cerning the optical part; though it is not my intention 

 to treat upon it in a regular way in this chapter, as a full 

 and particular account thereof will be given hereafter, 

 I shall speak of it now only as an appendage to the me- 

 chanical fabric of the engiscope, and the directions re- 

 lative to its use will be only of a general nature, but stiR 

 sufficient for practical purposes. The optical part, then, 

 is divided into the objective and ocular; the objective 

 glass is situated at 'e in figure 7? and screws on to the 

 neck of the body ; it is always next the object of which it 

 forms an image or picture, which is viewed by the ocular 

 or eye-glass at 'c, always situated next the eye, (for be it 

 remembered, that in strict language we do not view the 

 object itself in an engiscope, but its image or picture.) 



Several aplanatic object-glasses and eye-glasses ac- 

 company the instrument ; their powers or foci are 

 generally marked upon them ; but the high powers may 

 be known from the low ones by this simple rule, viz. 

 the largest object-glasses are the lowest in power, and 



