274 Mr. H. Hennessy on the Application of Photography 



stance, while the front and back should each consist of a thin 

 film of glass equal in breadth to the bore of the tube. In the 

 absence of such an instrument I used a spirit thermometer, 

 the tube of which had a large bore and a small external dia- 

 meter. This was inserted in a slit made in the side of a flat 

 box, and the portions at each side of the spirit column were 

 blackened. A sheet of photographic paper, prepared with 

 bromide of potassium and nitrate of silver, was placed on a 

 plate of glass which was drawn by clockwork through the 

 box. The plate was at a distance of nearly •! of an inch from 

 the column of spirit in the thermometer, and five feet from an 

 open window. The window faced towards the east, but the 

 direct rays of the sun did not fall on the apparatus during the 

 experiment. As the plate passed before the thermometer, a 

 beam of light was thrown upon it more or less high, accord- 

 ing to the height of the spirit in the tube or the temperature 

 of the surrounding air. A shadow bounded by a defined 

 curve was thus formed, as is shown in the figure. 



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The imperfections of my apparatus having rendered thie 

 motion of the plate carrying the paper rather irregular, the 



