141 
There is great discrepancy between the curve and straight 
line, when the width of the strip is equal to a d, fig. 5, but 
the difference is inappreciable when limited to b c. 
The width of the active rays thus employed, is determined 
by a diaphragm, with shifting sides, placed at the back of the 
camera, near the surface of the plate, and advantage has been 
taken of this in order to produce clouds, and generally atmos- 
pheric effects not attainable in the ordinary camera. 
By making the aperture taper thus 
Fig. 6. 
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1 
Liu 
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the narrow portion being opposite the sky, it is obvious that 
a totally different exposure will be given according to the 
width of the opening, and directly proportionate to the extent 
of that opening. 
If the part (a) be fifty times the width of the part ( b), then 
the time of the exposure of the first plane of the picture and 
that portion of the sky which approaches the zenith will be 
as 50 to 1. 
Now we know that we can get the most delicate cloud by 
a sufficiently short exposure, and that we can also get parts 
n e 
