310 HANDBOOK OF PHOTOGRAPHY 



well as other filter holders, should be designed to hold the filters against the lens 

 secureh- but without pressure. 



Cementing Filters. — It is impossible for the average photographer having no optical 

 measuring devices and but little technical knowledge of the design of optical parts to 

 construct high-quality filters for photographic purposes; such filters had best be 

 purchased. Certain instances do arise, however (for instance, that in which a filter 

 is to be used in the path of a beam of dispersed light, as in photomicrography), where 

 the optical qualitj^ of the filter is secondary to its spectral-transmission character- 

 istics, and for such cases cemented filters can be made which will answer the purpose 

 satisfactorily. 



Such cemented filters use, as their main optical apparatus, the gelatin films, such 

 as are available from the Eastman Kodak Co. and known as Wratten gelatin films. 

 These are available in about a hundred different spectral-transmission characteristics 

 and densities and will be an adequate selection for almost any photographic purpose. 



It is also essential that good-quality cover plates be used. Plate glass of selected 

 qualitjs free from striae, bubbles, or other defects, is generallj^ used for protecting the 

 gelatin film. Glass which, when viewed along the edges with white-light illumination, 

 has green color is not suitable for good-quality filters, as this glass has appreciable 

 absorption at both ends of the visible spectrum. Normal white glass (as distin- 

 guished from glass which is made to appear white by adding decolorizers) should be 

 used, as the decolorizers merely introduce additional absorption in the green portion 

 of the spectrum, thereby making the entire glass approximately neutral. Glass 

 which has been made white by the addition of decolorizers might be used, but the 

 efficiency of the filter and the filter exposure factor will be affected. 



Canada balsam is the most common cement for optical parts. This is a mixture of 

 turpentine and resins and is used in its natural condition. Canada balsam is a thick, 

 viscous, yellow fluid, having much the appearance of honey. A good grade of filtered 

 balsam will be required for cementing optical parts. 



All cementing operations should be carried out in a dust-free room. The Wratten 

 gelatin film will be found to be clean when purchased, but the glass plates, which should 

 be the same size as the gelatin film or slightly larger, should be thoroughly cleaned by 

 washing in potassium permanganate to rid them of grease and then washing in clean 

 hot water. The plates may be dried by evaporation or may be dried with a soft 

 cloth, if care is taken to remove all lint before beginning the cementing operation. 



The Canada balsam may be used at ordinarj^ temperatures; it may also be thinned 

 by adding turpentine or alcohol. However, a much more satisfactory^ method is to 

 use the balsam without the addition of other ingredients, thinning the cement by 

 heating the balsam slightlj' in a hot water bath. When the balsam is thinned suffi- 

 ciently by the application of heat, a drop is placed in the center of one of the glass 

 plates bj^ means of a stirring rod, and the gelatin film is placed on top of this spot of 

 cement. The gelatin should then be flattened down, making sure that no bubbles are 

 left between it and the glass plate. Provided they are clean, the fingers may be used 

 to flatten the gelatin to the glass plate, especially if protected by lens tissue. A drop 

 of cement is then placed on top of the gelatin and the top plate is pressed down, 

 making sure that all air bubbles are excluded. It is important to use sufficient 

 cement so that some will flow out on all edges; too little balsam will ruin the job as 

 more cannot be added. Not a little technique and skill are required to perform these 

 operations satisfactorily, and good results cannot be obtained consistently; there is 

 bound to be some spoilage if any quantity of work is undertaken. 



When this sandwich has been prepared, it should be pressed together. There will 

 probably be a decided tendency for the plates to slide upon one another. This may 

 be counteracted by holding the edges in place and binding the sandwich of glass-gela- 



