853 
The correction of the resistance is calculated from the deviation in mm. 
The influence of the error mentioned under 3 has been minimised 
by this procedure, for: 
1. the thermopile is irradiated always i the same way, and during 
the same short time, 
2. every observation is the mean of several measurements. 
Besides every photographic plate is at least measured twice. 
Il. The measurements. The first observations showed at once that 
as was to be expected, the blackening is a function of the wave- 
length of the light with which it is measured. Frequently repeated 
observations confirmed the first results. 
The following plates were used: Wellington plates, Speed 100, 
and Speed 400, dimension 13 X 18 cm., always of the same emul- 
sion number *). 
Every period (1) and (3) of 10 seconds is succeeded by a period (2), in 
which the key Wp is in position (2); hence the galvanometer is protected by 
the shunt from too great deviations. - 
In this period (2) of ten seconds the necessary manipulations, as the opening 
and closing of valve Kp and key S/, the adjustment of the resistance box 
BK to the resistance approximated before, and the noting down of the 
observation, may be performed. 
Every observation consists in the reading of 3 zero-positions (1) and 2 
positions (3), and lasts therefore 90 seconds. The course of the observation 
may be characterized by giving the periods (1) (2) and (3) in the right 
succession, viz.: 
H=2=—3)—2)=H)=2—3)=@)—()- By averaging the 3 positions (1)and 
the 2 positions (3) and by subtraction, the sought small deviation is found 
in m.m. 
1) The plates Speed 100 are developed with hydroquinone according to 
the recipe: 
Ist solution: Hydroquinone 36 érms 
Potassium meta bi-sulphite 36 „ 
Bromide of potassium OR 
Water 3000 „ 
2nd solution: Potassium hydroxide N44 rhs 
Water 3000 5 
Used were equal volume parts of the two solutions. 
The plates Speed 400 were developed with glycin according to the recipe: 
Distilled water 3000 grms 
Sodium sulphite (powder) 150 „ 
Glycin 30 
Potassium carbonate tS Gere 
As fixing bath is used in both cases: 
Ist solution: Hyposulphite 1500 grms 
Water 3000 „ 
2nd solution: Ammonium chloride 600 „ 
Water 30005 
Mix A and B. 
