286 Scientific Proceedings, Royal Dublin Society. 
Lastly, if we place the object between f and the lens, we find 
that the rays diverge on the far side, and can only be 
made to appear to meet on the opposite side of the 
lens. Where they appear to meet, there is the image; 
it is virtual, larger than the object, and erect. 
Should we now desire to use the instrument for another lens of 
different focal length, we have only to move the nut representing 
the principal focus to the required position in its slot, and then 
clamp it there, and move the object as before. 
If we wish to employ the instrument for the demonstration of 
the properties of concave lenses, we have only to change the 
position of the principal focus, and bring it to the front of the lens, 
as seen in the figure (fig. 2). Now withdraw the wire passing 
through the former principal focus, and insert it in the moveable 
nut at the far side, and also in the extremity of the section of the 
lens. This gives us the new position of the parallel ray after 
refraction through a concave lens. Move the object backwards 
and forwards as before, and you can discover for yourself all the 
laws and properties of concave lenses. In this case you will find 
from the instrument that the image is always virtual, smaller 
than the object, and erect, no matter what the position of the 
object. 
Norr.—In the case of virtual images with convex lenses, in 
order to find their positions, the rod passing through the extremity 
of the lens and the principal focus must be slid through the holes 
in which it fits, and so brought to the front of the lens. Still, as 
before, where this line intersects the principal axis, there we have 
the image. 
This instrument, with very slight alterations, and worked on 
the same principles, illustrates and proves all the properties of 
concave and convex mirrors. 
3°.—LHxperimental Proof of the Laws of Lenses. 
The second end to be secured by the use of this instrument 
is to prove experimentally that the images really are formed 
at the points indicated. This is done in the simplest possible 
manner. Over the arrow, a source of light, such as a candle, is 
fitted ; over the section of the lens, a glass lens of the given focal 
