462 REFLECTION OF DIFFERENT SCREENS [CH. XII 



The cloth screens were not so satisfactory as the ground-glass 

 because the crossing threads make a kind of grating and one sees 

 diffraction images ; and if one is in direct line with the arc lamp 

 the cloth acts almost as if it were transparent. The translucent 

 mercerized paper used in making tracings is practically as good as 

 ground-glass, but it is difficult to hold it smooth and even. The 

 tracing cloth used by architects and engineers is good for a trans- 

 lucent screen. 



There is a practical difficulty with all translucent screens. On 

 account of the poor reflection, the operator cannot tell with the 

 same certainty when the image is in focus as with a white, opaque 

 screen. 



632. Table of the reflection of different screens compared 

 with magnesium oxide. 



No. At 15 At 45 At 60 



1 Magnesium Oxide 100 83 



2 Plaster of Paris 95.6 88.7 78 



3 Cardboard 84.5 67 



4 White Lead 88.5 79.4 



5 Century Company's White 89.4 81 



6 Zinc Paint 84.4 76.5 



7 Aluminum Paint on Card 210 18 



8 Zeiss Metallic Screen, smooth .... 136 14 



9 Mirror Screen 200 



10 White Muslin, Reflection 73.4 69.1 66.9 



Transmission 39 30 



11 Gauze, Reflection 33 27 



Transmission 35 



12 Ground-Glass, Transmission 300 14.2 



13 Bristolboard, Reflection 91.5 



631a. For example, in the anatomical institute at Munich. Here all the 

 projection, whether with the magic lantern, the projection microscope or the 

 opaque lantern, is upon a translucent screen; also in some of the lecture rooms 

 in Holland. 



