INTRODUCTION. 23 
which may be placed in the hands of the Heliotype 
Company. 
Photo-micrographs also furnish a ready means 
of illustrating popular lectures, by the projection 
upon a screen of highly magnified microscopic 
objects. 
These pictures from life have always a greater 
interest for those who are being introduced to the 
wonders of nature as revealed by the microscope, 
than have hand-made drawings. 
Positives upon glass suitable for projection are 
made very quickly, and without a camera, by 
simply placing a dry plate at the back of the 
negative, exposing it to the light for an instant, 
and developing the image with the ferrous- 
oxalate solution. 
