6g GRAY SUBSTANCE OF THE TRAPEZIUM, 



Process presenting difficulties which render it in some respects less suitable. In 

 using as low an objective as a three inch, the first difficulty encountered is in 

 adapting the collodion to a light which is so extremely brilliant as the direct 

 sunlight reflected in the manner described above, it may therefore be well to give 

 the formula used. In so brilliant a light not only is an ordinary collodion altogether 

 too rapid, it being almost impossible to cover the lens quickly enough, but the 

 resulting negative is excessively thin, and destitute of that intensity which is requi- 

 site in order to give a clear and brilliant print. 



The pyroxyline used was chiefly the French (Poulenc-Wittman), and the plain 

 collodion prepared as follows : — 



Alcoliol (.805) 10 ounces. 



Ether (.Y25) 10 " 



Pyroxyline 300 grains. 



The iodizing solutions were prepared as follows, the formulae being taken from 

 Hardwich {Manual of PliotograpMc Chemistry, 1861), 



No. 1. (Potassium lodizer.) 



Iodide of Potassium .135 grains. 



Alcohol (.816) . 10 ounces. 



No. 2. (Bromo- lodizer.) 



Bromide of Ammonium ....... 40 grains. 



Iodide of Ammonium . . . . . . . 90 " 



Iodide of Cadmium . . . . . . . 90 " 



Alcohol (.816) 10 ounces. 



Two separate portions of the plain collodion were iodized with No. 1 and No. 2 

 resj)ectively, in the proportion of two parts of iodizer for six parts of plain collodion, 

 and the iodized collodion mixed after a few days, in the proportion of J or J of 

 No. 2 with No. 1. The resulting collodion gave the best results after keeping from 

 two to six months ; it had then acquired a decidedly red color, and gave a thin, very 

 even film, giving pictures remarkably free from imperfections of any sort, and though 

 exceedingly insensitive for common purposes, requiring in the brilliant light of the 

 microscope an exposure of only 8 or 10 seconds, which is much more easy to manage 

 than any shorter time. The film was sensitized in the ordinary Nitrate 'Bath., p^-ejjared 

 with distilled ivater, the strength being about 40-45 grains. For developing the 

 picture, I have preferred the use of pyrogallic acid to the ordinary developer prepared 

 with sulphate of iron, as I have found it much more controllable than the latter, and 

 giving with more certainty the requisite degree of mtensity. I have prepared it as 

 foUows : — 



Pyrogallic Acid ........ l^ grain. 



Acetic Acid, No. 8 . . . . . . . .30 minims. 



Distilled Water ........ 1 ounce. 



The picture required no re-development, and I invariably made it a rule to throw 

 away any picture Avhich after the first development appeared deficient in intensity, 

 as any attempt to re-develop injured very much the finer details of the picture. 

 The fixing solution consisted of the usual saturated solution of hyposulphite of soda. 



I 



