o f Edin burgh , Session 1880-81. 
103 
taken to have tlie illumination homogeneous. This condition was 
secured by causing the light from each source to pass through a 
combination of blue glass and blue paper steeped in a solution of 
sulphate of copper. This combination of glass and paper was en- 
closed in two tubes. If the apertures were equal, the blue spots 
seen on admission of a source of light were exactly of the same 
tint and intensity ; but if one of the apertures were a little smaller, 
one spot not only seemed darker, but of a slight difference of colour. 
This peculiarity, when combined with a definite modification of 
the aperture of the tube next the source of light to be compared, 
enabled the observer to determine gradations of light with fully 
more exactitude than the method of equal shadows. 
Mr Ponton was much occupied with the laws of chromatic dis- 
persion, and read papers on that subject at the meetings of the 
British Association held in 1859 and 1860. At the latter meeting 
he also read a paper “ On the Laws of the Wave-Lengths corre- 
sponding to certain points in the Solar Spectrum.” Indeed, it is a 
remarkable circumstance, that at the time of his death, notwith- 
standing his advanced age, he was engaged in constructing an 
instrument for making apparent to the eye the different lengths of 
the waves of light emanating from two differently coloured media. 
In addition to the scientific papers enumerated, he wrote several 
treatises. His most important work is entitled “ The Beginning, 
its When and its How.” 
He endured his protracted affliction with exemplary patience, 
and endeared himself to all who knew him by his cheerfulness 
and thorough kindness of disposition. 
It was on the 3d of August 1880 that Mungo Ponton, who will 
ever be remembered along with Daguerre and Pox Talbot, as one of 
the fathers of photography, passed away. 
Thomas Knox. By the Hon. Lord Shand. 
Mr Thomas Knox was born at Greenlaw in Berwickshire in 1818. 
At an early age he left his father’s house and came to Edinburgh, 
where he was apprenticed as a draper. Soon after having completed 
his apprenticeship he went to Dundee, where he remained for some 
time as an assistant in an extensive warehouse. It was there he 
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