108 PRACTICAL PHOTOMICROGRAPHY. 



color difficulties, that we can only give the outlines of the 

 principles upon which we work. In the majority of cases a 

 color-correct plate is almost useless unless we use in conjunction 

 with it a " screen," but it must be remembered that if we are 

 using an oil light, and particularly a paraffin lamp without 

 camphor in the paraffin, our light is yellow, and to a 

 considerable extent acts precisely as a yellow- screen would do. 

 It is necessary to have several yellow screens of different tints, 

 and it is well to have also one or two screens of blue, also of 

 varying tints. A glass of a color known as " signal green " 

 will be found useful for pale reds of which eosin stains are 

 types. The colored glasses called " screens " ought to have 

 their sides perfectly parallel, (ought to be what opticians call 

 "worked,") but unless the screens are to be used in some 

 critical position in the light rays, as for example close behind 

 the condenser, we do not insist upon this perfection. We do 

 not find that it matters in the least where the screen is placed 

 so long as it is between the light and the object, and so long as 

 no light reaches the object except such as passes through the 

 screen. The colored glass may be put in a holder about midway 

 between the light and the condenser. If, as in some cases, the 

 screen is screwed into a part of the mounting of the condenser 

 (in which position it is often called a " light-modifier "), then 

 it ought to be " worked " glass. A set of most useful screens 

 can be made by mixing aurantia with collodion, and three 

 screens may advantageously be made in this way by mixing 

 the aurantia in three different proportions, about one grain of 

 dye being first dissolved in one ounce of alcohol. The col- 

 lodion may then be poured upon a glass plate, which has 

 been previously thoroughly cleaned, and then rubbed all 

 over with French chalk (powdered talc), the chalk being 

 apparently all removed with a clean dry cloth. If desired, the 

 collodion, which must be in an even film, may be, after drying 

 stripped from the plate and used as a film, or the edges may 

 be varnished, and the film left on the glass. One screen should 

 be of the very palest color, the other two progressively darker. 

 At need any two, or all three, may be used together. We are 

 indebted to Mr. C. H. Bothamley for the suggestion of aurantia. 



