Prof. Wallace's Description of the Eidograph. 439 



passing an axis through the tubes of P the copying point of A, 

 and T" the tracing point of B. If this axis contained a copying 

 pencil, it would make a reduced copy of the original, and P', the 

 copying point of B, would at the same time make a copy of this 

 first copy ; if the intermediate copy be not wanted, it need not 

 be made. If each instrument were adjusted so as to make a 

 copy, reduced in the proportion of five to one, the copy made at 

 P' would be one twenty-fifth of the original. It is evident that 

 two or even more reduced copies might be made at the same 

 time from one original, by using several instruments. I have not, 

 however, made more than one at a time by two instruments. It 

 was in this way that some of the etchings on copper referred to 

 in this paper were made ; their sizes were ■§-, \, \, and T V of the 

 original The place of the copying pencil was supplied by a steel 

 point, which just cut through the varnish of the etching ground ; 

 this was all that was required to allow the acid to act on the 

 copper. 



