1886.] 



MICROSCOPICAL JOURNAL. 



85 



be made considerably cheaper by the 

 use of the carbon process than by 

 using dry phites. The process is 

 very cheap and not difficuh of apph- 

 cation. Prepare a sokition of gehitin 

 in distilled water, which when warm 

 shall be quite fluid, not much thicker 

 than milk. In this, while warm, 

 place a small piece of bichromate of 

 potassium, and shake the mixture till 

 it acquires a strong yellow color ; 

 then remove the undissolved bichro- 

 mate and add enough thick indian 

 ink well rubbed up in water to render 

 the emulsion so black that when held 

 between the eye and a gas-jet large 

 print cannot quite be read but can be 

 just dimly distinguished. 



Now take plates of clear glass cut 

 to proper size and thoroughly cleaned, 

 warm them and flow them with the 

 black gelatin emulsion so that a thin, 

 ev'en coating is left upon the glass. 

 Any excess may be poured back, just 

 as wet plates are coated with collo- 

 dion. Dry these gelatin plates in the 

 dark, keeping them level until the 

 film has set, when they may be placed 

 on edge and the drying finished. 

 They are now sensitive to light and 

 must be kept and handled like dry 

 plates, which in fact they are, only 

 less sensitive than the bromo-gelatin 

 dry plates generally used. 



To use these carbon plates they are 

 placed in a printing frame under a 

 negative and exposed to sunlight for 

 about the same time required for 

 silver prints, and when removed from 

 the printing frame in the dark room 

 are first immersed in cold water, then in 

 warm, and after two or three minutes 

 soaking are held under a gentle stream 

 of quite warm or moderately hot water, 

 which speedily dissolves and washes 

 away the gelatin forming the lights 

 of the image, while the shadows hav- 

 ing been acted on by the sunlight, 

 are insoluble and but slightly lose 

 their color in washing. When the 

 image is sufficiently clear the plate 

 is rinsed in cold water and dried in 

 the light. 



The lantern slides when dry are 



mounted and used precisely as those 

 made upon the ordinary dry plates. 



Paper coated with the same emul- 

 sion will give prints in the same way, 

 and such are the 'carbon prints' so 

 much used in England and on the 

 Continent, but paper is more diffi- 

 cult to manipulate than glass — at least 

 I have found it to be so. 



Lantern transparencies when pre- 

 pared to show microscopic objects 

 verv highly magnified are best made 

 from camera enlargements of a less 

 highlv magnified negative, as follows : 

 Prepare a negative showing the de- 

 sired points by means of an objective 

 of as low power as will clearly show 

 all the desired details. This negative 

 will be smaller than is required but 

 will be a better one than one made of 

 the desired size by a higher power, 

 because the penetration of the objec- 

 tive — however much abhorred pene- 

 tration is of real value here — will 

 give sharper projection than if a 

 higher power were used. Place the 

 negative in a copying camera and en- 

 large it to the desired size if possible, 

 if not a second enlargement would be 

 required, but is seldom if ever neces- 

 sary. The second plate, that is the 

 enlargement of the first negative, is a 

 positive, and if well done may be 

 mounted as a lantern slide, but first 

 a negative is made from this by con- 

 tact printing, and from this negative 

 not only paper prints but other lan- 

 tern positives may be made at will. 

 It shoidd be noted that if any retouch- 

 ing in the original negative is required 

 it must be done with care and skill as 

 any errors would be exaggerated by 

 the enlargement, but the enlarged 

 positive may be freely retouched be- 

 fore being used for contact print- 

 ing, and thus letters, figures, names, 

 etc., may be introduced into the 

 lantern slides prepared from the 

 last negative, which also may be 

 retouched if necessary like any other 

 negative. The superiority of such 

 enlargements over negatives made 

 originally of the same size is often 

 very marked. 



