202 Col. Waterliotise — Beversal of image on Photogra])7nc plates. [Aug. 



Thio-carbamide appears to have a more powerful reducing action 

 on the bromide and chloride than the two alcoholic thio-carbamides, 

 and in strong solution has been found to darken an ordinary gelatine 

 dry plate in the dark even without the aid of alkali, though it does not 

 visibly darken precipitated silver bromide. With iodide it is less 

 active. 



Thio-sinamine and thio-cavbamide in watery solution, especially the 

 former, seem to exert a solvent action on the silver haloids. 



Urea does not visibly darken or reduce the haloids in the dark, even 

 with alkalis. 



Phenyl-thio-carbamide seems to reverse the image most satisfac- 

 torily when used with the ordinary eikonogen developer in the propor- 

 tion of from 20 to 25 parts of the saturated solution in 100 parts of 

 mixed developer. The presence of sodium sulphite seems an advantage 

 but may be dispensed with ; a little bromide of potassiam improves the 

 result. With ferrous oxalate, pyrogallol and quinol developers the same 

 reversino- effect has not been obtained as with eikonogen. 



Allyl-tJdo-carhamide appears to be a much stronger reversing agent 

 than the phenyl salt, probably owing to its greater solubility, and rever- 

 sals Lave been obtained by its means with the pyrogallol and quinol 

 developers (containing sodium sulphite) as well as with eikonogen, but 

 not with ferrous oxalate. About 1 part of the saturated solution in 100 

 parts of developer is sufficient to produce the effect. 



Thio-carbamide has not yet been fully tried ; it evidently has strong 

 reversing powers, but works irregularly and must be used in very minute 

 proportions, otherwise no reversal is obtained. 



Urea used with the eikonogen developer shows no tendency to 

 reversal, whether added in the same small proportions as the thio-carba- 

 mides or in larger ones. It has not yet been tried with other developers, 

 but the result will probably be the same. 



With the phenyl-thio-carbamide and thio-sinamine the image 

 generally developes first of all quite normally and then gradually re- 

 verses, and sometimes a positive image is visible on the plate before 

 fixing. At others the action is more irregular and no image is seen 

 until the plate is fixed. Some of the positive images thus obtained 

 have a very rich brown or purple colour and are dense and full of detail, 

 thouo-h not always perfectly reversed nor clear in the lights. They are 

 quite different to the weak gi'ey images ordinarily produced by solari- 

 sation or over-exposure, which, in this case has to be very carefully 

 avoided. 



With regard to the cause of the reversal he had not been able to 

 give this part of the question full attention, nor in any case would it 



