Mau. 1S97.] BOTANICAL SOCIETY OF EDIX1U;I;GII 47 



remain in a plane at right angles to the optic axis of the 

 apparatus. The camera is now extended as far as necessary 

 to produce the required magnification and cover the plate, 

 this distance, of course, varying with the lens used. 



FocusiXG. — The Welsbach is turned on, and the first 

 focusing done by means of a ground glass focusing- 

 screen. It will now be seen if the whole of the surface is 

 satisfactory, and if any alteration in the position of the log 

 is deemed necessary to get the best surface in the field, or 

 to get the surface all in the same place at right angles to 

 the optic axis of the apparatus, such alteration is now to 

 be made. A focusing-screen of transparent glass is now 

 substituted for the ground glass one, and the finer focus- 

 ing is done by means of the special focusing-lens ; it is at 

 this point that one regrets any carelessness in the matter 

 of placing the log, either its position or stability. For 

 micrographic work, focusing with the transparent screen 

 is a necessity, and here it must certainly not be omitted, 

 as it is impossible to bring out the fine surface detail 

 without it — the ground glass screen is merely used to 

 ascertain that all is in satisfactory position. 



The Welsbach is now turned into the bye-pass, and the 

 magnesium ribbon is lighted, and fed slowly and as 

 steadily as possible through the ribbon tube. "While 

 an assistant attends to the lighting, the final focusing for 

 the intense magnesium light is made, and the necessary 

 stop inserted in the lens. A considerable time may 

 profitably be spent in the operation of focusing. I often 

 spend half an hour over it, and thereby save much 

 subsequent disappointment. 



The Eadiant. — This is placed as near the surface as 

 possible, but care has to be taken that the light shall not 

 " flare " the lens. This latter trouble may be obviated by 

 means of a hood of paper or metal painted mat-black 

 placed over, and extending some way out from, the front of 

 the camera towards the surface of the log. This prevents 

 any flaring of the lens, and allows of the illuminaut being 

 brought as near as may be desired. 



On the average, a distance of eight inches from the 

 light to the illuminated surface was found suitable. In 

 regard to the angle at which the rays of light from the 



