Popular Science Lectures. 161 
with a salt which, on exposure to light, renders the gelatine 
insoluble. The parts of the picture which remain un- 
affected are reduced or removed by washing, and from 
the image in relief thus obtained a mould is taken in 
which copper is deposited by the ordinary electrotype 
process, and the resulting block used in the printing 
press. Thus we are enabled to draw and engrave without 
pencil or burin — almost without touch of hand. 
Perhaps no greater scientific marvels have arisen 
during the century, than those derived from a closer 
knowledge of the properties of light. The discovery of 
certain rays called actinic rays and their effect on 
chemical substances gave us photography. No art has 
become more popular — few more useful. Apart from the 
service rendered in preserving the likeness of absent or 
departed friends, or in making known to us the features 
of those who are creating the history of our day, in 
bringing before us places and scenes we have visited, or 
others we may never see, it also is the means of 
producing, at little cost, copies of pictures and statuary, 
and so contributes to that greater appreciation of art 
which has sprung up during the last 40 years. 
Photography is also a valuable means of noting the 
progress of works of construction, and so constituting a 
periodical and unerring record. It is besides this em- 
ployed as an ever-watchful never tiring recorder of 
astronomical phenomena. Clever mechanical appliances 
have more recently made it available for taking instan- 
taneous portraits without the knowledge of the sitter and 
this adaptation is useful for police purposes. 
At present, as you are aware, photographs are mono- 
chromes — all of one colour. Attempts to reproduce the 
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