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that the individual parts begin to get confused and indistinct ; this 

 magnifying cannot, however, be effected at once. In the small nega- 

 tives the lights and shades are the reverse of what they are in nature, 

 consequently a print on pap?r therefrom gives the light and shade 

 correct. A photographic copy of a negative, however, produces a 

 positive by transmitted light, and a print from this would have the 

 shadows light and the light parts dark ; consequently, in magnifying 

 a negative with the intention of still producing a negative, an inter- 

 mediate transmitted positive must first be taken, and this in its turn 

 magnified, when it will produce a negative. 



The relative distances of the negative and focusing glass from the 

 intermediate lens were so adjusted, that an image of the negative, 

 enlarged to about two diameters, was thrown upon the ground glass, 

 care being taken that the light from the sky was reflected parallel 

 through the centres of the negative and lens by means of the mirror . 

 The aperture of the lens was then stopped down to half an inch by 

 means of a diaphragm, and the focus most carefully obtained by 

 sliding the end of the large camera in or out. I found it necessary 

 to verify this by experimental trials at different distances on each 

 side of the observed focus, as it was difficult to judge accurately with 

 the eye on the ground glass, owing to the roughness of the latter and 

 the feebleness of the light. 



A picture, or rather many pictures, were now taken, and the one 

 which by transmitted light most truthfully resembled a paper print 

 from the small negative was reserved for further magnifying. This 

 was effected absolutely in the same manner as the former : the nega- 

 tive being removed and the positive being placed in its stead, a further 

 magnifying gave a large-sized negative. 



Although this process seems very simple, it is impossible to esti- 

 mate the difficulties, unless by an actual repetition of the experiment, 

 which I had to overcome before arriving at the beautiful result which 

 I have the honour to lay before the Society. The double copying 

 had a tendency to slightly exaggerate the effect of light and shade, 

 and this could only be obviated by exposing the plates for such a 

 time, that with the feeble light at my command it was verging on 

 decomposition ; particles of dust, too, seemed most pertinaciously to 

 fix themselves on the prominent mountains, giving rise to craters 

 where none should be ; and even my finished pictures are not per- 



