208 



THE AMEEICAN MONTHLY 



[November, 



But there is an easy escape from the 

 difficulty by means of the well-known 

 ferro-prussiate process. Let the cam- 

 era lucida drawing be made with 

 good black ink directly upon tracing- 

 paper placed over white paper, or it 

 may be drawn on white paper and 

 carefully traced on tracing-paper. In 

 either case, when dry, gum the tracing- 

 paper on a sheet of clean glass, with 

 the drawing next the glass ; it may 

 then be printed from as an ordinary 

 negative, and any lettering will be re- 

 produced in its pi-oper position. 



With suitably-prepared ferro-prus- 

 siate paper in clear sunlight it prints 

 quicker than an ordinary gelatine neg- 

 ative ; from lo to 15 seconds will suf- 

 fice for well-made drawings on clear 

 paper. The copies are thus made 

 very quickly in any number, and are 

 rinsed in water and dried much more 

 quickly than an equal number of sil- 

 ver prints could be toned and fixed. 

 Fine details are well reproduced by 

 this method, which, though an old 

 one, seems to have fallen into disuse, 

 except by engineers and architects, 

 who still use it for reproducing 

 plans, etc. 



In a future paper I will give some 

 formulae for rapid printing paper, 

 and a method by which waste devel- 

 oper may be used for the production 

 of paper suitable for proofs, etc., and 

 some 'wrinkles' in the line ot pho- 

 tography as applied to microscopical 

 purposes. ^ ^^ y^^^^^ 



[The plan proposed by Mr, Vorce 

 is an excellent one, although it does 

 not dispense with the necessity of 

 camera lucida drawings. As this 

 seems to be a desideratum, we add 

 a few words which may be found 

 useful by some readers. Instead of 

 making a camei-a lucida drawing, take , 

 a negative from the object in the 

 usual way and make a blue print on 

 the ferro-prussiate paper. The draw- 

 ing may then be traced from that on 

 the tracing-paper, which would, 

 doubtless, be a better plan for those 

 who cannot use the camera well ; 



but the following plan, which we 

 would suggest for trial by some ex- 

 perimenter, is even better. Mr. Voixe 

 promises a formula for the ferro-prus- 

 siate paper, so we will not give one 

 at present. Our plan is to coat very 

 thin and transparent paper with the 

 ferro-prussiate solution, and print 

 from the negative upon that. Then 

 draw the outlines and necessary de- 

 tails on the print with India ink. 

 Having done this, bleach out the blue 

 picture with very dilute ammonia, 

 which will leave the paper white, 

 with the black, ink lines intact. 

 Should there be a yellowish color 

 left on the paper, a little weak acid 

 will remove it. After the paper is 

 washed and dried it may be spread'on 

 a flat, heated plate — a flat-iron, for 

 example — and paraffin rubbed over 

 it. This will make the paper trans- 

 parent, — like the 'wax-paper' used 

 by the confectioners to put up cara- 

 mels, etc. — and it can then be at- 

 tached to a glass plate, as suggested 

 by Mr. Vorce. — Ed.] 



The Use of Chinese Ink in Micro- 

 scopy.* 



BY M. LEO ERRERA. 



An object is seen in the microscope 

 more or less clearly as its index of re- 

 fraction and its color differ from the 

 index of refraction and color of the 

 surrounding medium. Thus, to ren- 

 der more apparent delicate details of 

 structure we place the objects in a 

 very refractive medium, or we cause 

 them to absorb appropriate coloring 

 matters. ' 



Certain objects permit coloring 

 matters to penetrate them with diffi- 

 culty ; others reject them entirely ; still 

 others permit the reagent to traverse 

 them readily, but do not retain the 

 color. In such cases we may resort 

 to a process the inverse of that habitu- 

 ally employed — color the surrounding 

 medium instead of the object. Hof- 

 meister recommends this method for 



[* Translated for this Journal from SulL Soc. 

 Beige de Micr.] 



