42 X. Vladimír Novák: 



with an internňtting source. This fact is said to be in a connection 

 with tbe photographie induction. 



Each sort of plates normály develloped is characterised by the 

 .,opacity~curve u according to Hurteb and Dbifeield, who constructed 

 such curves from the product / T. (intensity by tirae) as abscisse 

 and from D (opacity) as ordinate. The opacity-curve is convex to the 

 X-axis the plate being underexposed, it changes then into a straigt 

 line which corresponds to the right exposure and bends to the X-axis 

 going through a maximum. 



From the three existing théories, explaining the effect of the 

 light on the photographie plate, the subhalloid-theory is recently 

 most probable; the observations and measurements of Lucgin and 

 Lutheb prooving the chemical theory for plates containining silver- 

 halloid without the cementing medium seem to be a good claim for 

 the subhalloid theory. 



I tried to find a new method of measuring the effect of the 

 light on the photographie plate especially such one, which would 

 not ueed the develloping of the plate. It is a well known fact that under- 

 exposed or the overexposed plate can be develloped in to a normal 

 negative, the Photometrie method in such cases cannot be correct. 



It is possible to measure the effect of light on the photographie 

 plate by the observing the changes of the electric conductivity of the 

 wet emulsion exposed to the light, or by measuring the potenciál- diffé- 

 rence between two halves of one plate, one half being exposed to 

 the light, the other being in darkness. The measurements of these 

 two kinds were complète;! by a série of measurements of the con- 

 ductivity of develloped plates in order to be able to compare the 

 electric methods with the Photometrie one. 



The plates ušed (6X9 cm) for a connected série of experiments 

 were eut from one large plate (18X24 cm ore 24 X 30 cm), then 

 put in destilled water for 20 minutes. The hanging drops of water 

 were thrown off by means of a centrifugal machine; two tin-electrodes 

 (0'2m» thick) were put on two oposite endes of the plates, follded 

 backwards, the plate cowerd with an equalsized glas-plate and immersed 

 for one moment, with its lenght into melted paraffin to close the 

 emulsion completely. 



Measuring of the résistance. 



I. The galvanic résistance of the plates was measured by means 

 of allternating currents with the Wheatstone's bridge, using a very 



