OBSERVING WITH AND MANAGING ENGISCOPK. 211 



aplanatics. The body turns round upon a swivel, by 

 means of the pinching screw, to suit the convenience of 

 the observer ; and the only real peculiarity in observing 

 with the reflecting engiscope is, that the optical part is 

 always at right atigles to that of the refracting one in any 

 given mode of mounting or observation — just as the eye- 

 tube of the Newtonian telescope is relatively to that of a 

 refractor, cat eris paribus ; but this instrument, as well as 

 the simple and compound microscopes, are probably best 

 on their appropriate stands, of which full and particular 

 descriptions will be given hereafter*. It is only for the 

 sake of those who should like to have the three instru- 

 ments jumbled together, that they are adapted to the 

 same stand in the present case. 



Courteous reader, I have endeavoured to supply the 

 place of a viva voce lecture on the instrument, and to in- 

 fuse into thee such knowledge as I possess, touching 

 the management of the aplanatic engiscope, &c. The 

 directions given will, I hope, be found more explicit, 

 clear, and intelligible, as well as more full, particu- 

 lar, and diffuse, than any others hitherto given in print. 

 Vale ant quantum valere possunt. By their assistance thou 

 shalt be enabled to enter into a course of researches very 

 nearly as profitable to thyself and fellow creatures as if 

 thou wert engaged in the sublime and important occu- 

 pation of determining whether the small star of e Bootis, 

 is of a greenish blue, or bluish green ; or whether some 

 nebula is very gradually or very suddenly much brighter 

 in the middle. 



* Vide Micrographia, chap. i. 



