268 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
or opaque, whichever is deemed best. Where the shadows of the rod fall, I cut out a 
square mask to enable me to view the two shades without distraction to my eyes by 
glare from adjacent parts. It will be seen that, as the light and the reflected beam 
are stationary, the method of varying distance cannot be adopted to vary the intensity 
of the light. To obtain the necessary variation, I employ revolving sectors. These 
sectors, being connected with a small electro-motor wheel, work with four Grove’s 
cells. The aperture of the sectors can be increased or diminished during motion by a 
simple arrangement. This is an admirable plan of graduating light, and answers for 
all purposes of the sort. It will now be apparent that, should the light vary, the 
results will not be vitiated in the least, since the original light is made to act as the 
comparison-light as well.” 
By this arrangement a “ density curve ” for the exposures given to the plate 
through the clear water was constructed, and the values of the other exposures 
through the turbid medium, in terms of the exposure through the clear water, were 
determined. A screen with squares of different translucencies, which had been 
carefully measured, was placed in front of the plate. 
The following is an example :— 
6-inch turbid water. 
Opacity of screen. 
Equivalent exposure through clear cell. 
39-5 
~ 9 
29-0 
= 6-5 
22-0 
= 5-0 
90-5 
20-5 
Or it required 4'415 times the exposure through the 6-in. turbid cell to be equivalent 
to a unit exposure through the clear cell. 
4-inch turbid water. 
Opacity of screen. 
Equivalent exposure through clear cell. 
39A 
= 14 
29-0 
= 11-5 
22-0 
= 8-0 
90-5 
33-5 
Or it required 2TO times the exposure through the 4-in. turbid cell to be equivalent 
to a unit exposure through the clear cell. 
In the above example, for convenience sake, the clear cell was exposed for two 
minutes through rotating sectors having an aperture of 30°. 
The 4-incli turbid cell was exposed for five minutes through rotating sectors having 
an aperture of 45°. 
