il4 THE AMERICAN MONTHLY [April 



essary, a light filter of a color complimentary to that of 

 the object; provided the latter is colored, as are histolog- 

 ical or pathological sections or stained bacteria. Ortho- 

 chromatic plates only, are suitable for photomicrography. 

 Of these I have obtained best results with the Cramer 

 rapid " Isochromatic." These plates when properly de- 

 veloped give excellent contrast and gradation. 



Of the different reducing agents hydrochinone, either 

 alone, or combined with eikonogen or metol, preferably 

 the latter, is best for bringing out the latent image. 

 Hydrochinone is slow in action but has the quality of 

 producing clearness and contrast. Metol is more rapid 

 and when used with hydrochinone starts the develop- 

 ment and brings out the detail quickly — density being 

 gained afterward by the combined action of it and the 

 hydrochinone. A most important element in a formula 

 for photomicrographic development is potassium bromide. 

 This salt has the quality of preventing chemical fog, of 

 somewhat restraining development, and of causing the 

 details to appear in the relative order in which they have 

 been produced by light intensity. With no potassium 

 bromide and with a developer reasonably strong in alkali, 

 all parts of the image, even those least impressed by light, 

 appear practically together. With bromide added, this 

 action may be modified from slight to a great retardation 

 of the less impressed parts according to the amount of 

 the bromide introduced. Practically, about one-half 

 grain to the ounce of mixed developer is sufficient to re- 

 strain development, to cause the gradations to appear in 

 proper order, and to prevent ciiemical fog even during 

 prolonged development. 



The complete formula is as follows : 



No. 1. Water, hot, distilled or boiled 250. c. c. 



Sodium sulphite 200 grammes. 



Potassium bromide 0.5 " 



Hydrochinoue 1.5 " 



Metol •. 1.5 " 



Cool before using. 



