26 Dr. Woods on the- probable Nature of the Sun's Body, 



image was produced ; and the size of the picture of the solid was 

 not influenced by the length of time of exposure. I at first 

 thought a very shght difference of size was apparent in pictures 

 produced by hirgely different lengths of time the plate was acted 

 on, but I found on examination that the appearance was produced 

 by the gases employed to heat the lime. I mention the circum- 

 stance in order to guard others from the mistake. 



Taking into consideration all the experiments I have made, I 

 have no doubt that the light from the centre of flame acts more 

 energetically than that from the edge on a surface capable of 

 receiving its impression ; and that light from a luminous solid 

 body acts equally powerfully from its centre or its edges ; and 

 therefore conclude that, as the sun affects a sensitive plate simi- 

 larly with flame, it is probable its light-producing portion is of a 

 similar nature. 



In the experiment I have just spoken of, I used a sensitive 

 surface prepared according to a formula which I am anxious to 

 publish, as I believe it will be found more sensitive, and perhaps 

 more easily manageable than any other hitherto known. I have 

 by means of it taken a very good picture of a building, on a 

 bright day, in as short a time as it was possible to uncover and 

 cover again tbe aperture of the camera with the hand. The 

 length of the focus of the lens was 6 inches, its aperture f ths of an 

 inch in diameter. The process differs from the usual collodion 

 one in substituting a mixture of iodide and chloride of iron for 

 iodide of potassium, and using collodion having in solution some 

 common salt. In 1844 I first introduced the iodide of iron as 

 a photographic agent. I found it at that time the most sensi- 

 tive I could procure, and since then, whether in processes on 

 paper, or in albumen or collodion on glass plates, I have always 

 succeeded with it better than w ith any other, both as to rapidity 

 and facility of use. Mr. Fox Talbot has found it sufficiently 

 sensitive when employed with Mr. Hunt's discovery of sulphate 

 of iron, to produce instantaneous results. The details of the 

 process are as follow : — 



Take of Sulphate of iron 40 grains. 



Iodide of potassium ... 24 grains. 



Common salt 6 grains. 



Spirits of wine or alcohol . 2 oz. 



Mthev 2 drachms. 



Strong water of ammonia . 3 drops. 

 Powder the salts and mix them well together, add the alcohol 

 and aether, and finally the ammonia. Allow the precipitate to 

 subside. For preparing the plate, mix one part of the clear 

 solution ^iik three parts of collodion, to which has been added 



