A New Erecting Camera Lucida. By E. M. Nelson. 25 
squares in the grating at the diaphragm will diverge from those in the 
image formed by the objective, and if a corresponding drawing be made 
on rectilinearly ruled paper a barrel distorted image must necessarily 
be produced. 
It is only when there is a pincushion distorted image in the Micro- 
scope that the method of drawing on rectilinearly ruled paper by 
comparison with ruled squares at the diaphragm of the eye-piece yields 
a true picture. 
Note. — Figs. 2 and 3 are not diagrammatic, but are drawn to scale, 
and if they are reproduced precisely the same size as the drawing they 
will represent the amount of distortion produced by a plano-convex lens of 
2f in. focus, such as the plano-convex field lens of Powell’s 2 in. 
Huyghenian eye-piece. If that, or a similar lens, be held about 17 in. 
from the eye and 6 in. from the pincushion figure, the convex side of 
the lens facing the figure, an inverted, real, but rectiliu ear image will 
be seen ; this proves that the lens has produced a corresponding amount 
of barrel distortion. Whereas, if the small barrel figure be brought 
in. from the lens, the eye being kept at the same distance as before, 
and the convex side of the lens still facing the figure, a virtual, erect, 
and rectilinear image will be seen, which shows that the lens has given 
pincushion distortion to an equal extent. The images will appear 
about the same size, the real being a diminished, and the virtual a 
magnified image. 
