CH. VIII} 



PHO TO-MICROGRAPHY 



209 



FIG. 172. Tripod magnifier as focusing glass. This 

 is carefully focused on a scratch or pencil mark on the 

 lower or ground surface of the focusing screen. Then 

 whenever the object is sharply focused the focal plane will 

 be at the level of sensitive surface. 



As shown in Fig. 169, the object is placed upon a 

 glass support and the background is quite a distance 

 below the support. For a dark object the background 

 should be light, and for a light one dark. Black 

 velveteen is excellent for a back-ground. The advan- p IG r y 2 



tage of the glass support is that the shadows in the 



background which often make it difficult to tell just where the specimen ends and 

 the background begins, is wholly done away with, and that too without at all 

 affecting the proper light and shade of the object itself. (Method of W. E. 

 Rumsey, Canadian Entomologist 1896, p. 84). 



FIG. 173. Focusing Glass. "It is achromatic, 

 consisting of a double convex crown lens and a nega- 

 tive meniscus flint lens cemented together." It screws 

 into the brass tube and is thus adjustable, enabling one 

 to focus the pencil mark in the clear area of the focus- 

 ing screen (Fig. 174} with great accuracy. It also 

 serves to focus the image with ease and accuracy. The 

 eye must not be too close to the upper end ofthefocus- 

 ing glass or the field will be restricted. (Gundlach 

 Opt. Co.) 



FIG. 173. 



FIG. 174. Ground-glass focusing screen 

 with central transparent area for exact 

 focusing with a focusing glass when one 

 does not possess a clear focusing screen. (/) 

 The ground surface ; (2) Central part with 

 oblong cover-glass and Canada balsam on the 

 ground surface to render it transparent. X. 

 The central point in the entire focusing 

 screen. It is made with a black lead pencil 

 on the ground surface. The focusing glass 

 is focused on this cross, then when the image 

 is in focus it will be at the level of the sen- 

 sitive coating of the plate . 



