CH. I] TROUBLES WITH THE MAGIC LANTERN 57 



or starch process is one of their great drawbacks. Only powerful 

 radiants give satisfactory screen images. 



93. Shadow on the screen with water-cell. In case the water 

 in the water cell has evaporated in part there will be a very dis- 

 agreeable shadow on the lower part of the screen (fig. 31). It is on 

 the lower part of the screen although it is the upper part of the water 

 cell that will be empty. This is due to the inverting action of the 

 objective. 



FIG. 31. SHADOW ON THE LOWER PART OF THE SCREEN WHEN THE 

 WATER-CELL is BUT PARTLY FILLED. 



S Screen image with shadow on the lower side. The water is of course 

 present in the lower part of the water cell, and absent from the upper part; 

 but, owing to the inversion produced by the objective, the screen image shows 

 the shadow on the lower part. 



Occasionally the water is entirely absent from the water-cell. 

 Then there will be a very poor screen image, the entire screen being 

 affected by the obscurities on the dry surfaces of the water-cell. 



BREAKING OF CONDENSER LENSES 



94. It is usually the lens next the radiant that cracks or 

 becomes shattered. This is due to the too rapid heating or cooling 

 of the condenser lens, or to the mounting, which may be too rigid 

 to allow of free expansion of the lens as it becomes hot. 



Condenser lenses are especially liable to break: (i) When too 

 heavy currents are used; (2) when the lamp-house is not well and 

 evenly ventilated; (3) when currents of cold air strike the hot 

 condenser; (4) when the lens mounting is not provided with 

 ventilating openings for free circulation of air between the lenses; 



