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distance of objects being known) their size could be measured ; 
or their size being known their distance. 
The author, also explained the mode of obtaining the 
exact measurement of objects, by laying divided scales 
lengthways and across, so that any person could verify the 
measurement from the photograph without risk of error. 
This had been suggested to the Astronomer Royal for Scot- 
land, Professor Smyth, who was at present in Egypt, taking 
measurements of certain portions of the interior of the great 
Pyramid, and extracts from some of his letters were read, 
approving the same; the illumination of the objects being 
accomplished by the use of Magnesium. 
