656 



SUMMAEY OF CURRENT RESEARCHES RELATING TO 



adopted in the case of more ambitious instruments, particularly those 

 which are intended for field work. 



Fig. 139 



Silk-Mercer's Microscope. — This instrument, for determining the 

 quality of silk and other fabrics, is made by several opticians, the 

 particular one shown in Fig. 140 being that issued by Messrs. 

 Swift. 



Fig. 140. 



Its general form sufiiciently appears from the figure. In addition 

 to the usual rack-and-pinion focussing movement, the body can be 

 moved from front to back and vice versa by a second rack and pinion, 

 lateral motion being obtained by shifting the whole instrument. 



In the projecting piece in front of the base slides a lower plate, 



