74 Intelligence and Miscellaneous Articles. 



I had intended to communicate much earlier the details of this 

 experiment at the Royal Institution, hut was prevented from doing 

 so at the time ; and soon afterwards I went on the Continent in 

 order to observe the total solar eclipse of the 28th of July. This 

 most interesting phenomenon I had the pleasure of witnessing at 

 the little town of Marienhurg, in the north-eastern corner of Prussia. 

 The observations will appear, I believe, in a forthcoming volume of 

 the Transactions of the Royal Astronomical Society. Among other 

 things, I was enabled to make a satisfactory estimate of the degree 

 of darkness during the total obscuration ; which proved to be equal 

 to that which existed one hour after sunset the same evening, the 

 weather being during that evening peculiarly serene, so as to allow 

 of a just comparison. 



This Continental journey having effectually interrupted my photo- 

 graphic labours, I have only recently been able to resume them. I 

 shall therefore now proceed to describe to you exactly the mode in 

 which the plates were prepared which we used at the Royal Institu- 

 tion ; at the same time not doubting that much greater sensibility 

 will be attained by the efforts of the many ingenious persons who 

 are now cultivating the art of photography. And it is evident that 

 an increased sensibility would be as useful as an augmentation in the 

 intensity of the electric discharge. 



The mode of preparing the plates was as follows : — 



1 . Take the most liquid portion of the white of an egg, rejecting 

 the rest. Mix it with an equal quantity of water. Spread it very 

 evenly upon a plate of glass, and dry it at the fire. A strong heat 

 may be used without injuring the plate. The film of dried albumen 

 ought to be uniform and nearly invisible. 



2. To an aqueous solution of nitrate of silver add a considerable 

 quantity of alcohol, so that an ounce of the mixture may contain 

 three grains of the nitrate. I have tried various proportions, from 

 one to six grains, but perhaps three grains answer best. More ex- 

 periments are here required, since the results are much influenced 

 by this part of the process. 



3. Dip the plate into this solution, and then let it dry sponta- 

 neously. Faint prismatic colours will then be seen upon the plate. 

 It is important to remark, that the nitrate of silver appears to form 

 a true chemical combination with the albumen, rendering it much 

 harder, and insoluble in liquids which dissolved it previously. 



4. Wash with distilled water to remove any superfluous portions 

 of the nitrate of silver. Then give the plate a second coating of 

 albumen similar to the first ; but in drying it avoid heating it too 

 much, which would cause a commencement of decomposition of the 

 silver. I have endeavoured to dispense with this operation No. 4, 

 as it is not so easy to give a perfectly uniform coating of albumen 

 as in No. 1. But the inferiority of the results obtained without it 

 induces me for the present to consider it as necessary. 



5. To an aqueous solution of protiodide of iron add first an equal 

 volume of acetic acid, and then ten volumes of alcohol. Allow the 

 mixture to repose two or three days. At the end of that time it 



