39 



points of two pins like a lever, and that these glasses are 

 moved and secured, not by the outer edges of the circular 

 plates of their frames, but by the small axes of those plates, 

 very near the centres of their motion ; whereas the mirror C 

 is fastened firmly on a broad plate, screwed tight near its 

 margin by 4 screws to the head of the index, it is not easy to 

 conceive how it can alter its adjustment ; though from the 

 above mode commonly in use for fixing the horizon glasses, 

 it is not unaccountable why they should frequently do so, not 

 being fastened by the margins of their frames. These frames, 

 however, are sometimes moved by endless screws playing ia 

 their ratched edges ; and this construction when well executed, 

 is much better than the former. 



Read 21st May, 1810. 



