Jan. 1899.] BOTANICAL SOCIETY OF EDINBURGH 163 



way. This has been shown to be possible for iodised and 

 bromised collodion plates, and also for gelatine emulsion 

 plates, by Carey Lea, Eder, Warnerke, Abney, and others. 



The origin of the image in this case is difficult of 

 explanation. Meldola (" Chemistry of Photography ") 

 suggests that as it has been proved possible to produce 

 minute chemical changes in compounds by mechanical 

 pressure, so it is " not improbable that the silver haloids in 

 presence of sensitisers should undergo a minute amount of 

 decomposition by strong mechanical pressure, the decom- 

 position being so infinitesimal as to be revealed only 

 on application by that most sensitive of micro-chemical 

 tests — the photographic developer." 



(c) The purely chemical method. — By alkaline solutions 

 of glucose and lactose, as well as alkaline hypophosphites, a 

 developable image may be obtained on the sensitive plate. 



By these methods, negatives may be obtained. 



(d) Direct positives can be prepared by solarisatiou. — 

 In 1859, Poitevin showed that it was possible to obtain 

 direct positives on glass, instead of negatives, by using 

 potassium iodide as an artificial solarising agent. An 

 iodised collodion plate is sensitised and exposed to diffuse 

 daylight for a few seconds, a film of " reduction product " 

 (Meldola) is formed on the surface of the unaltered haloid. 

 After washing to remove the silver nitrate, the plate is 

 coated with a film of potassium iodide, and a long exposure 

 is given. 



" The most strongly illuminated portion of the film 

 becomes rehalogenised, while the deep shadows remain as 

 unaltered reduction product, and the intermediate shades 

 get partially rehalogenised. On development, the high 

 lights therefore come out white, the shadows dark, and the 

 intermediate shades of an intermediate tint ; in other words, 

 we get a positive instead of a negative." — (^leldola, 

 " Chemistry of Photography.") 



Of these four methods, I propose to give in the present 

 paper a resume of my experiments and results in the first 

 two — the mechanical and photo-chemical. From the 

 former method, I have as yet obtained no results. Whether 

 my plates were not sensitive enough, or whether the 

 pressure applied was not sufficient, I cannot say, but, at all 



