8 Prof. Marshall Ward, On a Problem 



contrasted with the large blocks used by Dr Russell), or whether 

 the fact of these thin sections being pressed fiat between glass 

 plates, wrapped up closely in several layers of black and other 

 papers, prevents the access of air, are matters for further ex- 

 periment. Both might be causal factors. 



It should be noticed that in experiments Nos. 35 and 36 I used 

 Scots Pine and Larch, on " Ordinary " Ilford plates ; and that 29 

 days at 20 — 22° C. failed to give an image. In experiments Nos. 

 50 and 51, the same sections were again used, but on Ilford 

 "Special Rapid" plates, and at 35° C, and good images were 

 obtained from both in 72 days. 



Dr Russell had already found that the " Ordinary " plates will 

 not do, and that raising the temperature of incubation and 

 lengthening the exposure increase the effects, so that this ex- 

 periment confirms his results so far, and in one case (Scots Pine) 

 with a wood known to be very active. 



But my experiments, Nos. 44, 45 and 48, with Pitch Pine, 

 another very active wood, where the period of incubation was only 

 10 days, indicate that temperature is not always an important 

 factor, for the image obtained in experiment 48, at 20 — 22° C, is 

 but little if any fainter than that of No. 44 or 45 incubated for 

 the same period at 35° C. 



The determining factor of greatest importance appears to be 

 the freshness or otherwise of the wood, as shown by the truly ex- 

 cellent photograph obtained by eight days' exposure at 35° C. of a 

 shaving of moist Pine — e.g., in experiments No. 21 and 21'. Russell 

 had also observed that fresh wood is more active than old dry wood. 



It is clear, then, that the action on the photographic plate can 

 be exerted by thin sections, though the time required is longer. 

 Unfortunately for my purposes the sharpness of the image is as a 

 rule deficient, and consequently it is as yet impossible to discover 

 the exact seat of the local action of the wood cell- walls. 



Before passing to some experiments where other factors are 

 introduced, let us now examine the table with reference to woods 

 other than the resinous Coniferse. 



Experiments Nos. 8, 12, 28, 46 and 47 were made with Oak, a 

 wood which abounds in Tannins, but in whioh there is no recog- 

 nisable Resin. 



As No. 8 shows, a thin section of Oak, old and dry, gave but a 

 poor image in eight days at 20 — 22° C; and, as No. 12 shows, 

 little if any better results were obtained in 18 days at the same 

 temperature. 



Fresh shavings of Joiners' Oak, incubated for 10 days, however, 

 whether at 20 — 22° C. (experiment 47) or at 35° (experiment 46) 

 gave far better results ; but it must be noted that these shavings 

 were moistened, to facilitate the cutting, and then dried, and I 



