162 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. i.xiii. 



between the mucilage and the cellulosic wall may be due 

 to a great concentration of sensitive salt in that region, or 

 it may be due to the presence of some specially sensitive 

 salt not existing in other parts of the plant. The fact that 

 gaslight is enough to cause reduction of the silver salt, 

 points to this latter theory as being the more probable. 

 It also suggests the likelihood of halogens being held 

 in rather complex molecules in the tissues, in addition 

 to their existence therein, as simple salts derived from the 

 sea, and not yet further elaborated by tlie metabolic 

 processes due to the plant's activity. 



On Contact Negatives for the Comparative Study of 

 Woods. By If. A. Eobertson, M.A., B.8c. (With Plate.) 



(Read 12th January ISD'J.) 



The object aimed at was to devise a method by which 

 several large sections of woods might be obtained on a 

 single sensitive plate, which could be used either as a 

 lantern slide directly, or as a negative for giving, by 

 exposure, ordinary prints. The following conditions had 

 to be fulfilled : — the sections must be large enough to 

 show the diagnostic characters of the wood ; the magni- 

 fication, if any, must be uniform ; and there must be enough 

 detail so as to permit of projection or examination directly 

 with a two-inch objective. The difficulties to be overcome 

 are fairly obvious to all workers in photography. 



It may be well to recapitulate here some of the various 

 ways by which an image, capable of development as a 

 sensitive plate, may be obtained — 



(a) The photo-chemical process, in which the plate is 

 exposed to light, and the chemical processes thus started 

 are carried on Ijy further treatment with certain chemical 

 substances. — Unitiue preparations of the retina in the shape 

 of contact negative prints on paper by an application of 

 this process are described by Musgrove, in " Vvoq. Scot. Mic. 

 Soc," 1891-92. 



(h) Tlie mechanical pressure method. — By suificient 

 mechanical pressure an invisible image can be produced on 

 the sensitive ]jlate capable of development in the ordinary 



