430 Prof. Wauuace on the Invention of the Pantograph, 
ductions of a subject, of various sizes, on an etching ground on 
copper, and the process afterwards completed by an acid in the usual 
way, and impressions printed from the plate.* Ofcourse the figure 
is reversed in respect of the original. This limits the applica- 
tion, except the copy be made by a reversing instrument ; but in 
many cases this is a matter of no moment. I know of no at- 
tempt to do any thing of the same kind by the Pantograph. 
2. In estimating the value of the instrument, the economy of 
time by its use is important. This will vary according to the in- 
tricacy of the design to be copied. I have been assured by a 
skilful engraver who had used the instrument, that in ordinary 
cases the saving may be three-fourths of the time required by 
the methods used before its invention; and that, in more intri- 
cate subjects, the saving may be nine parts out of ten. 
3. Another advantage is, that a youth whose labour costs lit- 
tle, may be taught in a few days to accomplish what would re- 
quire years of practice, by the eye alone, in the ordinary way. 
4. In the last place, the work may be better done. In the 
old methods, its accuracy could only stand the test of the unas- 
sisted eye ; but if done by the Eidograph, it will bear an exami- 
nation by measurement on a scale. 
The principle which has directed the construction of the 
Pantograph and Eidograph is the same: it is taken from geo- 
metry, and may be stated thus: “ If two points move ona plane, 
so that the straight line which joins them passes always through 
a fixed point as a pole; while in every position the distances of 
the moving points from the fixed point have to each other the 
same invariable proportion: then if, by any means, one of the 
points is carried along any line, the other shall trace a line ex- 
actly similar; the parts of the two lines passed over by any 
change of position of the points, having always to each other the 
invariable proportion of their distances from the fixed point.” 
* Specimens of these etchings were deposited in the library of the Society some 
years ago. 
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