860 
cases it is no more than 2 or 3°/, owing to the many precautions 
taken in the treatment of the plates. One of these is that all the 
plates after having been fixed and rinsed for a long time, are once 
more rinsed with distilled water for 10 or 12 hours, before they 
are dissolved in nitric acid. 
The extent of this error is then so perfectly accidental, both for 
the gauging liquids and for the liquids under examination, that we 
have not taken it into account. ') 
§ 3. Measurements of the Cross Section of the Silver Grains 
in the Photographic Plate. 
The way in which the silver of the photographic plate acts on 
the light, and the blackening observed in consequence of this, depends, 
as is immediately seen, to a great extent on the way in which the 
silver is distributed. The dimensions of the silver grains with respect 
to the wave-length of the light used in the measurement of the 
blackening, is of the greatest importance, If it is, therefore, required 
to obtain results which are liable to theoretic discussion, it is 
necessary to investigate, besides the blackening and the total quantity 
of silver, also the distribution of the silver. In this we have con- 
fined ourselves to the determination of the mean size of the silver 
grains — for so far as this is possible — in the plates of which 
the blackening and the total silver content was investigated before. 
The measurement takes place with a Zeiss-microscope with oil- 
emersion of numerical aperture of 1.3C, combined with an ocular 
N°. 12. An ocular micrometer is gauged with a Zetss-object glass 
micrometer. Thus it appears that a scalar division of the ocular 
micrometer corresponds to 1.0 u. 
The value of the cross-section of the grain is the mean of 40 
observations. ”) 
With the smallest blackenings of the plates developed with hydro- 
quinone N°. 1, 2, and 3 the grain cannot be measured, it being so 
small, that nothing is to be seen in the microscope but a faint 
greyish tint. In plate 4 the grain bas been measured, but it is 
already very diffuse. Of the plates 5, 6, and 7 the size of the grain 
can very well be measured. For plate 8 — 0.49 u was found for 
the cross-section — the result was more or less uncertain on account 
') The quantity of silver of the collargol used has also been determined. 
*) Of these 40 observations 20 were always by Dr. H. C. BURGER and 20 
by me. The mean difference between the two series of observations amounted 
to 5%, 
