856 
In fig. 3 the ratio of the transparency for yellow, resp. ultra red 
to green has been plotted as ordinate against the blackening for 
green as abscissa. 
Hence it appears that for the region yellow-green the plates 
developed with glycin with a blackening of from O to 1.0 to 8°/, 
may be safely used as reducers. For the rest it is, however, advisable 
always to gauge the plates with regard to their transparency for 
the wave-length region for which they must be used. In the investi- 
gation by Miss Riwiin cited above this has been done by the method 
worked out by us’). 
The mean error of the observations of blackening has the value 
of 0,7°/, for blackenings of 2.0 to 0.1, the value of 3.3°/,7) of the 
blackening for blackenings of 0.1 to 0.001. Also with blackenings 
above 2.0 the accuracy becomes less. 
§ 2. Determination of the Quantity of Silver Present per Unit 
of Area on a Blackened Photographic Plate. 
1. Method of procedure. It appears from the results of § 1 that 
the quantity of silver present per unit of area on the photographic 
plate cannot be proportional to the blackening. 
The estimation of the silver was made with the extinction meter 
of Mo11.*) in the form as it was used in Mr. Dirmarscn’s still un- 
published investigation of the flaking of colloids. For particulars of 
the research we refer to the publication of Mr. Dirmarscn’s results. 
For our purpose it is sufficient to observe what follows. 
The deviation of the galvanometer caused by change of the turbidity 
in one of the dishes, is registered photographically *). Accordingly 
') In our investigation it is impossible to reproduce all the blackenings that 
are to be investigated, on one photographic plate. This is preferable especially 
in a search for the law of blackening, properties of the blackening etc. As 
the way in which the problem has been put, necessitates the determination 
of the quantity of silver of the plates, and this requires a black plate of 
sufficiently large area, we meet with practical difficulties when we wish to 
represent all the blackenings on one plate. The same investigations have, 
however, first been carried out with the different blackenings in small squares 
on the same plate. The results obtained were the same. Hence the differences 
found cannot be attributed to the use of different plates. 
*) These slight blackenings can be measured with the same apparatus by a 
somewhat more elaborate, but also more accurate way, which reduces the 
mean error to 0.9%, 
3) Versl. Kon. Akad. v. Wetensch. XXVIII 1920, p. 1001—1006. 
‘) The error resulting from the changes in the dimensions of the registering 
paper, which may appear after the development, appears to be smaller than 1 %, 
