PHOTOGRAPHY TO THE MICROSCOPE. 265 



higher at the time of the immersion of the cotton ; a more 

 prolonged washing than that prescribed in this work is de- 

 sirable. Tins collodion can be sensitized with advantage by 

 the iodide and bromide of cadmium, in the proportion of four 

 to one. A receipt published in ' Humphrey's Photographic 

 Journal/ has lately been used by me with very good results."^ 



No. 1. No. 2. 



Plain collodion, 1 oz. Plain collodion, 1 oz. 



Iodide of ammonium, 5 grs. Iodide of jjotassium, 5 grs. 



Bromide of potassium, 3 grs. Bromide of ammonium, 3 grs. 



Dissolve the iodide of ammonium and bromide of ammo- 

 nium in alcohol, the iodide of potassium and bromide of 

 potassium in the least possible quantity of water, before 

 adding them to the plain collodion. Mix Nos. 1 and 2 in 

 equal parts for use. 



This collodion, when used according to the wet process, 

 though not very intense when first made, is quite sensitive, 

 negatives of landscapes being obtained in a quarter of a second, 

 indicating by their strength that a shorter time would suffice. 

 It acquires intensity by keeping. The exposure is effected by 

 the flap at r, and will last from a quarter of a second to four 

 minutes, according to circumstances. The development is as 

 usual, hyposulphite of soda being used as a fixing agent. The 

 use of the bromide of arsenic, mentioned in the same journal, 

 gave, with some samples of collodion, excellent results, so far 

 as intensity was concerned. 



The negatives thus obtained are examined by a lens of 

 1-inch focal length, to test their degree of sharpness. This 

 quality will not only vary with the manipulation, but with 

 the nature of the object; the sharpest negatives obtained by 

 me, when examined by a power of forty diameters, appear as 

 well defined as finely executed lithographs seen by the naked 

 eye, while other classes of objects (dots in pine wood, &c.), 

 with all care, yield negatives wiiich present the same appear- 

 ance under a much lower magnifying power. 



Positive prints. — In order to preserve the fine details, the 

 prints should be taken on glass, not on paper ; mica answers 

 when a print is to be transmitted by mail. Great care should 

 be used that little or none of the fine markings on the nega- 

 tive are lost in this process ; a bright light (sunlight thrown 

 on the negative backed by ground glass), a small diaphragm 

 before the copying lens, and careful allowance for the chemical 

 focus, are the essentials. To produce enlarged positive prints 

 on glass, the negative is placed on the stage of the micro- 



* 'Humplirey's Journal,' Joseph H. Ladd, New York. 

 VOL. TI. NEW SER. T 



