246 BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX 



The light scattered in the room is liable also, if the room is finished 

 in a light tint, to diminish the brilliancy of the screen image by 

 lessening the contrast. 



To avoid the troubles just considered, the objective should have 

 a perforated hood over its front. The perforation should be of the 

 diameter of the front lens. The free surface of the hood over the 

 front of the objective should be perfectly flat, and should be finshed 

 in dull black (fig. 139-140). Such a hood is also of the greatest 

 use in enabling one to center the light ( 375, 372a). 



A B 



FIG. 140. END VIEW OF A HOODED OBJECTIVE SHOWING THE LIGHT 

 CENTERED AND OFF CENTER. 



In A the image of the crater is directly over the opening in the hood and 

 therefore gives the greatest light for projection. 



In B the crater image is at the right and only a small amount of light enters 

 the objective. 



In both A and B the negative or lower carbon is shown by cross lines. It is 

 above owing to the inverting action of the condenser. 



373. Light shield beyond the objective. There should be a 

 flat or concave shield beyond the objective to prevent any stray 

 light reaching the screen from the apparatus except what passes 

 through the objective (see fig. 133, 138). 



CENTERING THE PARTS OF THE PROJECTION MICROSCOPE ON ONE 

 LONGITUDINAL Axis 



374. For micro-projection it is absolutely necessary that all 

 the parts or elements should be on one straight longitudinal axis 

 like beads on a rod. With the large lenses used in magic lantern 



372a. If one docs not have the metal-hooded objectives (fig. 139), 

 ordinary, nickel-plated objectives can be greatly improved by painting the 

 bright surfaces with dull black ( 3/ia). The objectionable reflections can 

 also be prevented by tying black velvet or blackened asbestos paper around the 

 objectives. 



