OF SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
267 
the values from the formula I' = Ie Ma: , y being ’5192 and a '595, were— 
689, 413, 248, 148, 87; 
again a sufficiently close value to show that it may be safely used. 
§ XVI. Experimental Photographic and Optical Tests of the Logarithmic Formula. 
A great many tests with turbid water were made to ascertain if the same held 
good experimentally. The same photographic preparation as that already referred to 
was used. A cell 6 inches long and 4 inches wide was employed, and different 
portions of the same photographic plate were used for obtaining impressions of the 
light acting. The light was allowed to fall through the cell, containing clear water, on 
an isolated portion of the plate f-inch square. Eight different exposures were given 
to various portions of the plate to form a scale of density of deposit. Exposures 
were given to the other parts of the same plate to light from a constant source 
passing through the 4-inch and the 6-inch thicknesses of clear and turbid water. 
The plate was then developed with ferrous oxalate,* fixed, and dried. The density of 
deposit of each square was next measured by a plan which I have described in the 
‘Photographic Journal.’ 
§ XVII. Mode of measuring the Density of Deposit on a Photographic Plate. 
From the paper m question I have made the following extract :— 
“ The light, whatever it may be, is placed at A; a lens L, at distance of its 
equivalent focus, in this case 9 inches. The negative, N, is placed in front of 
this, and another lens, L n , throws the image on the screen S, in front of which is a 
rod, R, whose shadow is cast by the light coming through L„. 
“ This is the ordinary optical lantern form of apparatus. At one side, at a convenient 
distance, I place a mirror, M, with the angle so adjusted in azimuth that it reflects 
the light from A over the patch illuminated by the lens L„. This naturally throws 
another shadow of the rod alongside the first shadow. If desirable, I can place the 
other lenses, L m and L IV , the latter forming the image of L Itl , upon the first patch of light. 
(As a rule, these last two lenses are unnecessary.) The screen, S, may be transparent 
* Tliis developer was used for convenience, as it gave a black deposit, which was useful for subsequent 
measures. 
2 M 2 
