27 



far distant from the head of the pin, that the ring will not 

 touch it, nor affect its motion or position. 



Tlie cock c and its cap are formed with an indenture in. 

 them, (as in the fig.) at the end of the index-gUiss, in order 

 that the mirror C may He nearer to it, which will allow the 

 little mirror to be made broader, Avithout enlarging the brass 

 plate A and the head of the index ; while the part of the 

 cock not indented may, as well as its wing -E, be mad^ so 

 massive and strong, as not to be bent and strained easily by 

 any accident : a notch is cut in the aide of the bar E at the 

 screw/, to allow this screw to be turned, without touching 

 the bar: thus both the mirrors may be adjusted independent 

 on each other. 



The little mirror C requires not to be silvered on the back, 

 and consequently its opposite surfaces need not be parallel, so 

 that it may be made of a piece of well polished and plane 

 looking glass, but the polish must be taken away from its 

 back surface by grinding it on a plate with fine emery and 

 water ; and the surface thus made rough should be smeared 

 over with a feather dipt in oil of turpentine mixed with 

 lampblack, to prevent all reflection from that surface. 



The addition of this mirror to the index adds no trouble to 

 the business of fixing the index-glass: the extra work re- 

 quired is that of the little mirror and of the plate d ; and the 

 fabrication of this plate will be greatly facilitated, if it be 

 cast from a model, in which the slits and perforations, and 



E 2 



