48 TRANSACTIONS AND PP.OCEEDINCS OF THE [Sess. lxi. 



radiant must fall upon the surface to obtain the best 

 results, one must take into consideration the character as 

 to colour, markings, etc., of the surface. A little experience 

 soon enables the operator to surmount this difficulty. In 

 the case of such surfaces as that of the oak and so forth, 

 which present comparatively well-marked colour contrasts 

 and distinct markings, the best results were got w^ith the 

 incident rays falling on the plane of the surface at an 

 angle of 40 degrees or so. On the other hand, surfaces 

 such as those of beech and birch, etc., with line lines and 

 markings, require the angle to be somewhat narrowed. 

 Alterations in the incident angle and the distance of the 

 illuminant, require to be made when working with highly- 

 polished surfaces of recent wood or of silicilied stems. 



I use no condenser for the light. At first I used two 

 bull's-eye condensers in an attempt to parallelise the rays, 

 but discarded both for two reasons : one was the difficulty 

 of ensuring that the unsteady light from the burning ribbon 

 should pass through both lenses and illumine the desired 

 part of the field, and the other was the difficulty of getting 

 a large enough field of light of uniform intensity to cover 

 the surface. I now prefer the simpler method of using the 

 ribbon as near the surface as I can get it, and giving a 

 longer exposure. 



Strong objection has been taken to the using magnesium 

 ribbon as an illuminant, because of the difhculty of ensuring 

 a steady and constant light. This difficulty has not made 

 itself very apparent in this method, as the results testify ; 

 and this, even with the very long exposures necessary, in 

 which many inches of ribbon were burnt, and the variation 

 in the light must of necessity have been at its maximum. 



Plate and Exposuhe, — Each worker has his favourite 

 plate, but after trying a number of diffi3rent plates of 

 different speeds, I always came back to the llford when I 

 wished to be certain of my results. Most of the work was 

 done with llford ordinary, but, in some cases, to overcome 

 colour difficulties I used the Isochromatic plates of this 

 maker. As far as my experience goes, a slow plate is to 

 be preferred for this special purpose of opaque wood 

 ndcrography. 



After the final focusing and stopping down of the lens. 



