00 THE BOOK OF THE LANTERN. 



doubtedly the best thing of all is a simple white-washed 

 wall. Why should this be so ? Let me endeavour to 

 explain the matter in a few words. 



Suppose that we go behind an ordinary linen or cotton 

 sheet, while lantern pictures are being thrown in front and 

 upon it. We not only see the picture distinctly through 

 the sheet, but there is enough light round about us to enable 

 small print to be read with ease. At least such is the case 

 with the lime-light, and in a minor degree with the oil 

 lantern. Now all this light means so much deducted from 

 the light available from the lantern, and which, therefore, 

 is completely lost to the spectators in front. From an 

 opaque wall, on the other hand, the light is nearly all reflected 

 to the spectators' eyes ; and although no doubt some must 

 be absorbed, we know that none is actually transmitted 

 through the screen, and utterly wasted, as in the case of a 

 semi-opaque sheet. We might compare the two cases, to a 

 vessel of water with a porous bottom, which will, of course, 

 allow a large portion of the liquid to dribble through and 

 be lost ; as against a vessel with a solid bottom which will 

 hold water without any wasteful transmission. But a 

 white-washed wall is not often met with in a lecture-hall, 

 and the best substitute is a canvas sheet rendered opaque 

 with white-wash. Such a sheet is portable up to a certain 

 size ; and if intended for use as a fixture in any one hall, 

 can be made up to any size, within reasonable limits. 

 Witness the scenes on rollers at our large theatres and opera 

 houses, the basis of which is simply whitened canvas. Such 

 a plan was adopted at the late Polytechnic Institution, where 

 the screen measured no less than 26 feet across. 



