202 



THE AMERICAN MOJSTHLY 



[November, 



going to be satisfied with silhouettes 

 made with a dark lantern attached 

 to a camera box.' Even the electric 

 light is condemned by this writer, 

 who says : • The incandescent lamp 

 is of no use, except as pi^eviously 

 indicated, for very unambitious at- 

 tempts with low powers.' 



There is no mistaking the author's 

 meaning in this regard. Nevertheless, 

 we venture to express an opinion 

 more or less opposed to his ; for not 

 only is it true that photographs of 

 delicate objects of all kinds have been 

 made,by artificial light, perfectly sharp 

 and satisfactory in every way, but it 

 is possible that lamp-Hght may be 

 found in certain cases to 1)e even 

 better than sunlight. But the practi- 

 cability of using artificial light is fully 

 demonstrated by the excellent work 

 of the Hon. J. D. Cox on diatoms, 

 with powers of 1000-1500 diameters 

 and more. Mr. W. H. Walmslev, 

 who is also an experienced worker in 

 this field, and who gave some emi- 

 nently practical demonstrations of 

 photo-micrographic work at the meet- 

 ing of the A. A. A. S. last summer, 

 which have been highly commended, 

 will fully confirm the value of arti- 

 ficial light for this purpose. As for 

 the incandescent electric light, we 

 need only refer the reader to the work 

 of Dr. Van Heurck, with powers of 



iV °^' I's"^^^^"* ^^^ ^^ ^^^ article on 

 the Electric Light in Microscopv, 

 published last )'ear. (Vol. v, p. 222.) 

 It shall be our endeavor, in writing 

 these articles, to make them eminent- 

 ly practical. Only well-tried and 

 reliable methods will be given, and 

 not one that the writer cannot fully 

 recommend from personal experience 

 with it. While we hope to make the 

 directions and explanations so clear 

 that any person can follow them and 

 perform the operations, yet it is de- 

 sirable that the beginner should spend 

 some time in the developing room 

 with an experienced operator to see 

 the methods of working. 



* This Journal, current volume, p. 44. 



In order that the beginner may 

 work intelligently it is advisable he 

 should know something of the chem- 

 istry of photography. The sensitive 

 plate is a piece of glass (or other 

 material) coated on one side with 

 emulsion. The emulsion is ^Dre- 

 pared by dissolving silver nitrate in 

 a solution of gelatin in water, and 

 adding thereto potassium chloride, 

 bromide or iodide, or a mixture of 

 these. The result of this addition, 

 which must be made in a room lighted 

 with ruby-glass windows, is to throw 

 down, or precipitate, very finely di- 

 vided particles of silver chloride, 

 bromide or iodide, since these silver 

 compounds are quite insoluble. 

 These minute particles remain sus- 

 pended in the gelatin. It is these 

 minute particles that are sensitive to 

 light, and from this time until the 

 plate is exposed in the camera, and 

 the picture is fully developed, the 

 emulsion must be carefully protected 

 from all except red light, to which 

 the particles are not sensitive. The 

 emulsion thus prepared is cooled, 

 the gelatin solidifies, and is then 

 thoroughly washed in water. It is* 

 then melted, flowed over glass plates 

 set perfectly level, and allowed to 

 dry, thus making a thin, hard coating 

 which is the sensitive film. 



The degree of sensitiveness de- 

 pends upon many conditions, but 

 primarily upon the silver compounds 

 used. Thus the chloride is less sen- 

 sitive to ordinary daylight than the 

 bromide. The chloride is sensitive 

 to blue and ultra violet rays, the 

 bromide is also sensitive to green 

 and yellow light, and the iodide has 

 its special range of sensitiveness in 

 the spectrum. Advantage is taken 

 of these peculiarities for particular 

 purposes, hence we have gelatino- 

 chloride emulsions, gelatino-bromide 

 emulsions, etc. The ])lates generally 

 used for ordinary photographic work 

 are made with bromide emulsions 

 containing some iodide. 



When such a plate is exposed to 

 light in a camera, the particles t)f sil- 



