284 Scientific Proceedings, Royal Dublin Society. 
Now the Image-Finder is merely an embodiment of this rule 
in simple mechanism, and so it furnishes an easy and rapid 
means of drawing images, no matter how the position of the 
object may vary. 
To see this more clearly we have only to look at the figure of 
the instrument (fig. 2). Here we have the object, viz. the arrow 
AB. 
1. From the extremities of this arrow (viz. A and B), we draw 
secondary axes represented by the wires AA and BB, 
and these revolve on a pivot which represents the centre 
of the lens. 
2. From the extremity of this object or arrow we draw a 
straight line parallel to the principal axis. This is 
represented by the parallel wire AF. 
3. From the point # where this wire cuts the lens, we pass 
a straight wire 4# through the principal focus. 
4. Where this wire cuts the secondary axis AA there is the 
image, v.g. at A. 
To be brief, the Image-Finder consists of an arrow or object 
capable of moving backwards and forwards along two parallel 
wires, which represent the parallel rays proceding from the object. 
From the point at which one of these parallel rays meets the lens, 
proceeds a ray passing through the principal focus; this indicates 
the course pursued by the parallel ray after refraction. Lastly, 
from the extremities of the object pass two straight lines through 
