CH. IX] BLACK APPARATUS FOR MICRO-PROJECTION 245 



Wright (p. 194) in speaking of the necessity of a dull finish in the 

 interior of objectives says : "I may add here that some really good 

 lenses [objectives] when used with brilliant light such as projection 

 demands, give a "mist" over the image purely from flare, or reflec- 

 tion in the lens mount, and which is removed by careful blacken- 



ing." 



Finally, there may be a bright spot or "ghost" in the screen image 

 from the internal reflections of a shiny microscope tube, especially 

 if the tube is small. If an ocular is used this ghost usually dis- 

 appears. It can also be avoided by having the interior of the 

 microscope tube a dull black ( 37ia). 



Objective Hood 



FIG. 139. PROJECTION OBJECTIVE WITH 

 BLACK METAL HOOD. 



372. Hoods for projection objectives. Usually the ends of 

 objectives are tapering and finished in polished nickel, making them 

 veritable mirrors. As the image of the source of light spreads more 

 or less beyond the opening of the front lens upon this mirror surface 

 the dazzling light is reflected into the face of the operator, and also 

 more or less around the room. The operator is likely to be so 

 blinded by the reflections that he cannot see to focus properly. 



371a. When necessary, a person can give polished surfaces a dull finish 

 himself. A camel's hair artist's brush should be employed for the finer work. 

 For the dull finish, dead-black japalac thinned somewhat with xylene (xylol of 

 the Germans) toluene or turpentine answers well. 



Dull black may be prepared by adding to thin shellac varnish plenty of good, 

 dry lamp-black. After thorough shaking, this should be filtered through 

 gauze to take out any coarse particles. If the shellac is too thick the resulting 

 finish is more or less shiny, but if the proper mixture is used the surface will 

 be very dull, but not so smooth as the japalac. 



As the black surface wears off by use, the bright surfaces underneath are 

 exposed, and occasionally one should go over the apparatus and reblacken all 

 bright spots. 



