7ih; 



Professor Sir James Dewar 



[Jan. 21 



a plaiR'-sided water cell to prevent heating, is directed at a moderate 

 inclination on to the film, and the reflected lisht is focused on 

 the screen by means of a suitable lens. For visual examination 

 more diffuse lighting* is preferable, and this can be obtained 

 by placing a ground glass in the path of the incident beam at 

 n little distance from the film, or for large films a plain white 

 screen adjacent to the film should be suitably illuminated 

 either by diffuse daylight, or beam from a lamp. In either case a 

 black background must be provided to the film, to absorb the trans- 

 mitted light. 



AYith a smoothly Ijlown bull), very little distortion is produced, so 

 that a clear elliptical image is seen. The appearance of the film 

 at two stages of development is shown in the accompanying plate 

 (Plate I. ) A portable form is shown in Fig. 7. The bulb is mounted 

 in a black box provided with a window opposite and parallel to the 

 film. The Avindow is central with the film of the same width, but 



Fig. 



about twice the height. The lower half of the window is ground 

 glass, illuminated by an ordinary electric pocket lamp fixed in a small 

 box. The_ film is view^ed through the upper clear part of the 

 window. 



